Chapter 3 and 4 Flashcards

Mendel was successful for several reasons. He chose to work with a plant, Pisum sativum, that was easy to cultivate, grew relatively rapidly, and produced many offspring whose phenotype was easy to determine, which allowed Mendel to detect mathematical ratios of progeny phenotypes. The seven characteristics that he chose to study exhibited only a few distinct phenotypes and did not show a range of variation. Finally, by looking at each trait separately and counting the numbers of the different phenotypes, Mendel adopted a reductionist experimental approach and applied the scientific method. From his observations, he proposed hypotheses that he was then able to test empirically.

Genotype refers to the genes or the set of alleles found within an individual. Phenotype refers to the manifestation of a particular character or trait.

The principle of segregation states that an organism possesses two alleles for any particular characteristic. These alleles separate during the formation of gametes. In other words, one allele goes into each gamete. The principle of segregation is important because it explains how the genotypic ratios in the haploid gametes are produced.

Mendel's principles assert that the genetic factors or alleles are discrete units that remain separate in an individual organism with a trait encoded by the dominant allele being the only one observed if two different alleles are present. According to Mendel's principles, if an individual contains two different alleles, then the individual's gametes could contain either of these two alleles (but not both). Blending inheritance proposes that offspring are the result of blended genetic material from the parent and the genetic factors are not discrete units. Once blended, the combined genetic material could not be separated from each other in future generations.

The concept of dominance states that when two different alleles are present in a genotype, only the dominant allele is expressed in the phenotype. Incomplete dominance occurs when different alleles are expressed in a heterozygous individual, and the resulting phenotype is intermediate to the phenotypes of the two homozygotes.

The multiplication rule allows for predicting the probability of two or more independent events occurring together. According to the multiplication rule, the probability of two independent events occurring together is the product of their probabilities of occurring independently. The addition rule allows for predicting the likelihood of a single event that can happen in two or more ways. It states that the probability of a single mutually exclusive event can be determined by adding the probabilities of the two or more different ways in which this single event could take place. The multiplication rule allows us to predict how alleles from each parent can combine to produce offspring, while the addition rule is useful in predicting phenotypic ratios once the probability of each type of progeny can be determined.

Walter Sutton's chromosome theory of inheritance states that genes are located on the chromosomes. The independent segregation of pairs of homologous chromosomes in meiosis provides the biological basis for Mendel's two principles of heredity.

The principle of independent assortment states that alleles at different loci segregate independently of one another. The principle of independent assortment is an extension of the principle of segregation: The principle of segregation states that the two alleles at a locus separate; according to the principle of independent assortment, when these two alleles separate, their separation is independent of the separation of alleles at other loci.

Assuming autosomal inheritance and complete penetrance, recessive inheritance is indicated by the presence of affected offspring from two unaffected parents. If the trait is rare, frequently the recessive allele will be passed for a number of generations without the trait appearing in the pedigree. For rare recessive traits, the trait often appears as a result of mating between two close relatives. Finally, if the trait is recessive, all of the offspring of two affected parents will be affected. Assuming autosomal inheritance and complete penetrance, at least one of the parents of affected children should be affected (unless a new mutation has led to creation of a new dominant allele). The trait should not skip generations within a lineage. Two affected parents who are heterozygotes can have unaffected offspring. Unaffected parents do not transmit the trait to their offspring.

Recessive trait - Red hair would often appear in the children of parents who lacked red, when both parents would were heterozygous. In a mating between two red-haired parents, all of the offspring would be expected to have red hair. Dominant trait - Red hair would only appear in children if at least one of the parents had red hair because the child would have to inherit the red hair allele from a parent, who would therefore also have red hair. A mating between two red haired parents might produce some children with non-red hair. A cross between two non-red parents would produce only non-red offspring.

Useful characteristics • Are easy to grow and maintain • Grow rapidly, producing many generations in a short period • Produce large numbers of offspring • Have distinctive phenotypes that are easy to recognize Examples of organisms that meet these criteria • Neurospora, a fungus • Saccharomyces cerevisiae, a yeast • Arabidopsis, a plant • Caenorhabditis elegans, a nematode • Drosophilia melanogaster, a fruit fly

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A statistical method used to evaluate the role of chance in causing deviations between the observed and expected numbers of offspring produced in a genetic cross. The probability value obtained from the chi-squared table refers to the probability that random chance produced the deviations of the observed numbers from the expected numbers.

In the XX-XY system, males are heterogametic and produce gametes with either an X chromosome or a Y chromosome. In the ZZ-ZW system, females are heterogametic and produce gametes with either a Z or a W chromosome.

In organisms that follow this system, there is no cytogenetically recognizable difference in the chromosome contents of males and females. Instead of a sex chromosome that differs between males and females, alleles at one or more loci determine the sex of the individual.

In humans, the presence of a functional Y chromosome determines maleness. People with XXY and XXXY are phenotypically male. In Drosophila, the ratio of X chromosome material to autosomes determines the sex of the individual, regardless of the Y chromosome. Flies with XXY are female, and flies with XO are sterile males.

Males show the phenotypes of all X-linked traits, regardless of whether the X-linked allele is normally recessive or dominant. Males inherit X-linked traits from their mothers, pass X-linked traits to their daughters, and through their daughters, to their daughters' descendants but not to their sons or their sons' descendants.

Tortoiseshell cats have two different alleles of an X-linked gene: X+(non-orange, or black) and Xo(orange). The patchy distribution results from X-inactivation during early embryo development. Each cell of the early embryo randomly inactivates one of the two X chromosomes, and the inactivation is maintained in all of the daughter cells. So each patch of black fur arises from a single embryonic cell that inactivated the Xo, and each patch of orange fur arises from an embryonic cell that inactivated the X+.

Barr bodies are darkly staining bodies in the nuclei of female mammalian cells. Mary Lyon correctly hypothesized that Barr bodies are inactivated (condensed) X chromosomes. By inactivating all X chromosomes beyond one, female cells achieve dosage compensation for X-linked genes.

Incomplete dominance means the phenotype of the heterozygote is intermediate between the phenotypes of the two homozygotes. In condominance, both alleles are expressed and both phenotypes are manifested simultaneously.

In incomplete penetrance, the expected phenotype of a particular genotype is not expressed. Environmental factors, as well as the effects of other genes, may alter the phenotypic expression of a particular genotype.

Cytoplasmically inherited traits are encoded by genes in the cytoplasm. Because the cytoplasm is usually inherited from a single (most often the female) parent, reciprocal crosses do not show the same results. Cytoplasmically inherited traits often show great variability because different egg cells (female gametes) may have differing proportions of cytoplasmic alleles owing to the random sorting of mitochondria (or chloroplasts in plants).

a. XY with the SRY gene deleted—female b. XY with the SRY gene located on an autosomal chromosome—male c. XX with a copy of the SRY gene on an autosomal chromosome—male d. XO with a copy of the SRY gene on an autosomal chromosome—male e. XXY with the SRY gene deleted—female g. XXYY with one copy of the SRY gene deleted—male In humans, a single functional copy of the SRY gene, normally located on the Y chromosome, determines phenotypic maleness by causing gonads to differentiate into testes. In the absence of a functional SRY gene, gonads differentiate into ovaries and the individual is phenotypically female.

Joe must have inherited the hemophilia trait from his mother. His mother could have inherited the trait from either her mother (a) or her father (b). Because Joe could not have inherited the trait from his father (Joe inherited the Y chromosome from his father), he could not have inherited hemophilia from either (c) or (d).

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Yes. If a woman has a son with hemophilia, then she is a carrier. Any of her sons have a 50% chance of having hemophilia. Her sisters may also be carriers. Her brothers, if they do not themselves have hemophilia (because they survived circumcision, they most likely do not have hemophilia), cannot be carriers, and therefore there is no risk of passing hemophilia on to their children.

a. XX—1 Barr body b. XY—0 c. XO—0 d. XXY—1 e. XXYY—1 f. XXXY—2 g. XYY—0 h. XXX—2 i. XXXX—3

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