Exam 3: Chapter 9 Book Notes

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The transmission of traits from one generation to the next

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heredity

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The scientific study of heredity, that began in 1860 by Gregor Mendel

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genetics

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Mendel stressed that the heritable factors, today called genes, retain their _______ generation after generation

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individuality

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A heritable feature that varies among individuals within a population, such as flower color in pea plants or eye color in humans

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character

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A variant of a character found within a population, such as purple or white flowers in pea plants

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trait

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Referring to organisms for which sexual reproduction produces offspring with inherited traits identical to those of the parents. The organisms are homozygous for the characters under consideration

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true-breeding

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An offspring of parents of two different species or of two different varieties of one species; an offspring of two parents that differ in one or more inherited traits; an individual that is heterozygous for one or more pairs of genes

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hybrid

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A mating of two sexually reproducing individuals; often used to describe a genetics experiment involving a controlled mating

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genetic cross

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The true-breeding parents

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p generation

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the hybrid offspring of the p generation

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F1 generation

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The offspring of the F1 generation

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F2 generation

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Describe three generations of your own family using the terminology of a genetic cross

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The P generation is your grandparents, the F1 is your parents, and the F2 is you and any siblings

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an experimental mating of individuals that are heterozygous for the character being followed (or the self-pollination of a heterozygous plant)

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monohybrid cross

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An alternative version of a gene

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allele

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Having two different alleys for a given gene

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heterozygous

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having two identical alleles for a given gene

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homozygous

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The allele that determines the phenotype of a gene when the individual is heterozygous for that gene

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dominant allele

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An allele that has no noticeable effect on the phenotype of a gene when the individual is heterozygous for that gene

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recessive allele

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A general rule in inheritance that individuals have two alleles for each gene and that when gametes from by meiosis, the two alleles separate, each resulting gamete ending up with only one alley of each gene; also known as Mendel's first law of inheritance

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law of segregation

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A diagram used in the study of inheritance to show the results of random fertilization

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Punnett square

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The expressed traits of an organism

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phenotype

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The genetic makeup of an organism

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genotype

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How can two plants with different genotypes for a particular inherited character be identical in phenotype

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One could be homozygous for the dominant allele and the other heterozygous

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The chromosomes in a homologous pair-chromosomes that carry alleles of the same genes- are also called-

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homologs

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A specific location of a gene along the chromosome

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locus

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Alleles of a gene reside at the same _____ on homologous chromosomes

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locus

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A mating of parental varieties differing in two characters

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dihybrid cross

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Assortment in which the allele for seed color and seed shape are inherited together as they came from the P generation

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dependent assortment

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A general rule of inheritance that when gametes form during meiosis each pair of alleles for a particular character segregates independently of other pairs; also known as Mendel's second law of inheritance

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law of independent assortment

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Predict the phenotypes of offspring obtained by mating a black lab homozygous for both coat color and normal eyes with a chocolate lab that is blind from PRA

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All offspring would be black with normal eyes

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a mating between an individual of unknown genotype and a homozygous recessive individual

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test cross

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A trait being dominant does not mean that it is normal or more common than-

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a recessive trait

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Traits most seen in nature that are not necessarily specified by dominant alleles

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wild-type traits

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Refers to a trait that is less common in nature

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mutant trait

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______ is a dominant trait, but it is also a mutant trait

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freckles

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An individual who is heterozygous for a recessively inherited disorder and who therefore does not show symptoms of that disorder but who may pass on the recessive allele to offspring

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carrier

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What is the most common lethal genetic disease in the US?

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cystic fibrosis

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Is cystic fibrosis dominant or recessive?

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recessive

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Extra fingers and toes are examples of-

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dominant disorders

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A form of dwarfism in which the head and torso of the body develop normally but the arms and legs are short; dominant disorder

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achondroplasia

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Only _________ have achondroplasia

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heterozygotes; which means offspring has 50% chance of getting this

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Dominant alleles that cause lethal diseases are much _______ common than recessive alleles that cause lethal diseases

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less

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A degenerative disorder of the nervous system that usually does not appear until middle age

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Huntington's disease

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Because the allele for Huntington's disease is _____, any child born to a parent with the allele has a 50% chance of inheriting the allele and disorder

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dominant

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Mendel's two laws explain inheritance in terms of _____ that are passed along from generation to generation according to simple rules of probability

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genes

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A type of inheritance in which the phenotypes of the heterozygote and dominant homozygote are indistinguishable

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complete dominance

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A type of inheritance in which the phenotype of a heterozygote (Aa) is intermediate between the phenotypes of the two types of homozygotes (AA and aa)

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incomplete dominance

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Dangerously high levels in the blood; high build up of LDL

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hypercholesterolemia

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The ABO blood group phenotype in humans involves ______ alleles of a single gene

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three

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Inheritance pattern in which a heterozygote expresses the distinct trait of both alleles

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codominant

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The control of more than one phenotypic character by a single gene

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pleiotropy

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A genetic condition caused by a mutation in the gene for hemoglobin. The mutation causes the protein to crystallize, which deforms red blood cells into a curved shape. Such blood cells produce a cascade of symptoms that can be life-threatening

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sickle cell disease

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What is an example of pleiotropy in humans?

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sickle cell disease

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In most cases, only people who are _______ for the nickel cell allele have the disease

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homozygous

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At the organismal level, a heterozygote displays _________ for the sickle cell trait, with a phenotype between the homozygous dominant and homozygous recessive phenotypes

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incomplete dominance

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At the molecular level, the two alleles in sickle cell disease are _________; the blood cells of heterozygotes contain both normal and abnormal (sickle cell) hemoglobins

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codominance

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The additive effects of two or more gene loci on a single phenotypic character

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polygenic inheritance

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What is the opposite of polygenic inheritance?

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Pleiotropy

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A basic principle in biology stating that genes are located on chromosomes and that the behavior of chromosomes during meiosis's accounts for inheritance patters

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chromosome theory of inheritance

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In the absence of the gene _____, ovaries develop rather than testes

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SRY

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A number of human conditions results from sex-linked recessive alleles located on the _________

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X chromosome

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Recessive x-linked traits are more common in-

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males than females

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An x-linked recessive trait; bleed excessively when injured because they lack one or more of the proteins required for blood clotting

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hemophilia

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An x-linked recessive disorder that is characterized by a progressive weakening of the muscles and loss of coordination

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Duchenne muscular dystrophy

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The scientist Jean Baptiste Lamarck proposed that if an individual acquired a particular characteristic, such as strength from lifting weights, this characteristic would be inherited by the organism's offspring. This same idea is also part of a more ancient idea, the idea of __________.

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pangenesis

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Mendel's view of the mechanism of heredity was radically different from the prevailing view of the time because he saw heredity working through _____

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unchanging (immutable), heritable factors that were contributed by each parent and never mixed

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In a monohybrid cross, F2 refers to __________.

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the second filial generation, or the \"grandchildren\" of the original mating pair

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Imagine a particular character (such as flower color) that is determined by a single gene. If this gene is present in two forms, how can you tell which allele is dominant and which is recessive?

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Perform a cross between two true-breeding individuals and observe the trait or traits expressed by the F1 individuals.

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In a standard monohybrid cross between purple-flowered and white-flowered peas, what fraction of the purple-flowered peas in the F2 generation would you expect to be true-breeding?

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1/3

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An insect that has the genotype EeGGcc will have the same phenotype as an insect with the genotype _____

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EEGgcc

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In an individual of genotype Aa, where are the A and a alleles physically located?

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One allele is on one chromosome, and the other is in the same position (locus) on the homologous chromosome.

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Mendel's principle of independent assortment applies to the __________.

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independent assortment of alleles of one gene relative to the alleles of any other gene

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Assume that in cattle a spotted coat is dominant to an even coat, short horns are dominant to long horns, and the traits for coat type and horn length assort independently. In a cross between cattle that are each heterozygous for both traits, what proportion of their offspring are expected to have long horns?

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1/4

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You cross two fruit trees. One tree produces lemons with spiky leaves. The other produces limes with smooth leaves. Your F1 generation produces lemons with smooth leaves and spiky leaves. What are the genotypes of the parents? L = lemons; l = limes; S = smooth leaf; s = spiky leaf.

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LLss and llSs

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Imagine that long fins in zebrafish is a dominant trait. A breeder wants to set up a breeding program beginning with homozygous dominant long-finned fish. If she obtains a handful of the long-finned fish, how can she tell which, if any, of these are homozygous for the trait?

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Cross the long-finned fish with short-finned fish; if the offspring are all long-finned, the long-finned parent is homozygous. This cross is a testcross (the short-finned fish must be homozygous recessive), and having all offspring show the dominant trait proves that the long-finned parent is homozygous.

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In Labrador retrievers, a common breed of dogs, black coat is dominant to chocolate, normal vision is dominant to progressive retinal atrophy (PRA), and normal hip joint is dominant to hip dysplasia. All these genes assort independently. Two dogs that are heterozygous for alleles of all three genes are crossed. Using rules of probability (not a Punnett square), what is the chance that the first pup born to these dogs will be chocolate, have normal vision, and have normal hip joints?

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9/64 The question asks about three traits, not two.

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Two normal parents have three normal children: one son and two daughters. Their son and one of their daughters marry and also have normal children. Their second daughter, Mary, marries a man with a rare, recessive blood disorder. They have two children, and both children develop the blood disorder. What were the genotypes of Mary's parents?

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Either one of her parents or both of her parents were heterozygous for the trait. From the information provided, it is not possible to discern which of these is correct for sure. If one of Mary's siblings did develop the disorder, you could determine that both of the parents were heterozygous.

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Human genetic disorders _____.

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are most often recessive

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Why are lethal dominant alleles so much more rare than lethal recessive alleles?

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Lethal dominant alleles are harmful whether they are carried in homozygous or heterozygous form, so there is always strong selection against these alleles.

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Imagine you're counseling a couple who have undergone carrier screening for Tay-Sachs disease. The man is a carrier, and the woman does not carry the Tay-Sachs allele. How should you advise them?

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They should be informed that if they have a child, the child will not have Tay-Sachs disease but will have a 50% chance of being a carrier of the Tay-Sachs allele. Because the woman will transmit a normal allele to the child, the child will not be affected; however, there is a 50% chance that the child will inherit a Tay-Sachs allele from the father and thus be a carrier.

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Akin to urban legends, there are curious genetics legends—things like eye color being determined by one gene, with a brown eye allele being completely dominant to blue. The problem comes when simple myth meets the complex reality of how eye color and many other traits are transmitted. Why is the inheritance of so many traits difficult to explain using only Mendel's view of genetics?

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Mendel was correct for the traits he investigated, but his principles must be extended (not discarded) to explain many more complex patterns of inheritance.

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Hypercholesterolemia is a disorder in which blood cholesterol levels are elevated. The H allele is incompletely dominant to the h allele, with hh homozygotes having extremely high levels of blood cholesterol. A husband and wife are both Hh heterozygotes. What is the chance that their first child will have normal levels of blood cholesterol?

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1/4

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Snapdragons show incomplete dominance in their flowers. A pink snapdragon is crossed with a red snapdragon. What color(s) are the offspring?

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50% red, 50% pink

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There are over 100 alleles known for the gene associated with cystic fibrosis. With current technology, it is possible to determine exactly which allele or alleles is/are carried by a person. What is the maximum number of different alleles that any person can carry?

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2 This is the number of different alleles present in the population, but the question asks how many different alleles can exist within a single individual from that population.

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In lentils, the C gene has two different alleles. CSCS homozygotes have spotted seeds, CDCD homozygotes have dotted seeds, and CSCD heterozygotes have seeds with both spots and dots. This indicates that _____.

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CS and CD are codominant

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Two individuals decide to have children. The expected blood group genotypes are 50% of blood type A, and 25% each of blood types AB and B. What genotypes are the parents?

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IAi x IAIB In this case, 50% of the expected offspring would have blood type B.

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If the gene for seed color that Mendel studied exhibited pleiotropy, how might a green pea be different from a yellow pea?

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With pleiotropy, there would be other differences between the peas—for example, green peas may also be larger, ripen earlier, and be more sensitive to frost. Pleiotropy refers to one gene being associated with more than one phenotype, not codominance, which is described here.

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In breeding pure-breeding large and small strains of mice, you cross individuals of each strain and note that their offspring are intermediate in size. Two models (explanations) to account for this result are (1) that body size in these strains is due to one gene with alleles that show incomplete dominance and (2) that body size is a polygenic trait. How could you distinguish between these models?

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Intercross the F1 and see if the F2 contains three size classes (consistent with the incomplete dominance model) or if there is a range of sizes (consistent with the polygenic model).

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Our understanding of the role played by genes in many human characteristics is advancing rapidly in, for example, body size, performance on IQ tests, and personality traits. In this new genetic era, the role of the environment __________.

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is to work with genes in complex and often unknown ways in the development of these traits

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The chromosome theory of inheritance is based upon which of the following sets of observations?

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Genes segregate; chromosomes come in pairs

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In corn, blue kernels are produced by a dominant allele of a coloration gene, and white kernels are produced in individuals homozygous for a recessive allele of the same coloration gene. Another gene has two alleles for shape, with smooth kernels being dominant to wrinkled. A plant heterozygous for both genes is testcrossed (crossed to a homozygous recessive white, wrinkled strain). The testcross offspring consist of the following types: 1,447 blue smooth; 1,436 white wrinkled; 150 blue wrinkled; 145 yellow smooth. Explain the inheritance of the coloration and shape trait

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The coloration and shape genes are linked on the same chromosome.

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In Morgan's testcross of a gray-bodied, long-winged heterozygous female Drosophila with a homozygous recessive black-bodied, vestigial-winged male, the following offspring were obtained: 965 gray body, long wing; 944 black body, vestigial wing; 206 gray body, vestigial wing; 185 black body, long wing. Focusing only on the recombinant classes (gray body, vestigial wing and black body, long wing), the numbers of offspring of each type are similar (206 and 185). What accounts for the similar number of offspring of each recombinant phenotype?

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Crossing over between chromosomes is reciprocal, so whenever a recombinant chromosome of one type is produced, there's a recombinant of the opposite type that is also produced.

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Imagine that a mutant strain of Drosophila undergoes crossing over at half the normal rate. How would a genetic map prepared for this mutant differ from a genetic map prepared for a normal (wild type) fly?

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The order of genes would be the same in both strains, but the distances measured between genes in the mutant would be half those of the wild type

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The existence of rare XY individuals who are phenotypically normal women was instrumental in learning about human sex determination. Maleness is determined by the SRY gene found on the Y chromosome. How is it possible to be an XY woman?

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The SRY locus of the Y chromosome is deleted

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Vitamin D-resistant rickets is an X-linked dominant bone disorder. A man with this form of rickets marries a normal woman. What proportion of the couple's daughters is expected to have vitamin D-resistant rickets?

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100%

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A woman who is a carrier of hemophilia marries a man affected with hemophilia. What percentage of their sons and daughters is expected to have hemophilia?

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50% of sons and 50% of daughters

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Because it is passed essentially intact from father to son, Y chromosome research has been particularly useful in improving our understanding of __________.

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recent human evolution

Exam 3: Chapter 9 Book Notes

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