Ch 14 Review Questions

Genetics I Answered Review Questions 1. Who discovered the laws of heredity and with what specific organism was this person working? “Modern genetics had its genesis in an abbey garden, where a monk named Gregor Mendel documented a particulate mechanism of inheritance. The painting in the figure below, depicts Mendel working with his experimental organism, garden peas. Mendel developed his theory of inheritance several decades before the behavior of chromosomes was observed in the microscope and their significance understood.” (Text quoted from page 251 of the textbook)

2. What are alleles and where would they be located in a cell? “…alternative versions of genes account for variations in inherited characters. The gene for flower color in pea plants, for example, exists in two versions, one for purple flowers and the other for white. These alternative versions of a gene are now called alleles (see table below). Today, we can relate this concept to chromosomes and DNA. As noted in Chapter 13, each gene resides at a specific locus on a specific chromosome. The DNA at that locus, however, can vary somewhat in its sequence of nucleotides and hence in its information content. The purple-flower allele and the white-flower allele are two DNA variations possible at the flower-color locus on one of a pea plant’s chromosomes.” (Text quoted from pages 253 and 254 of the textbook.)

3. Contrast dominant and recessive alleles. “if the two alleles at a locus differ, then one, the dominant allele, determines the organism’s appearance; the other, the recessive allele, has no noticeable effect on the organism’s appearance.” (Text quoted from page 254 of the textbook) A dominant allele may be expressed when only one dominant allele is present. However for a recessive allele to be expressed two recessive alleles must be present. Each parent must be at least heterozygous for offspring to have a recessive allele expressed.

4. What is the Law of Segregation? “The law of segregation, states that the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes. Thus, an egg or a sperm gets only one of the two alleles that are present in the somatic cells of the organism. In terms of chromosomes, this segregation corresponds to the distribution of homologous chromosomes to different gametes in meiosis. Note that if an organism has identical alleles for a particular character —that is, the organism is true-breeding for that character—then that allele is present in all gametes. But if different alleles are present, as in the F1 hybrids, then 50% of the gametes receive the dominant allele and 50% receive the recessive allele.” (text quoted from page 254 of the textbook) 5. Contrast genotype and phenotype. “Because of the different effects of dominant and recessive alleles, an organism’s traits do not always reveal its genetic composition. Therefore, we distinguish between an organism’s traits, called its phenotype, and its genetic makeup, its genotype. In the case of flower color in pea plants, PP and Pp plants have the same phenotype but different genotypes [see figure below].” (text quoted from page 256 of the textbook)

6. Explain how to do a single-character cross. Identify the character: Let’s say we are interested in Huntington’s Disease. HD is caused by a dominant allele (A).

Identify the phenotypes of the parents: Father has HD; Mother is normal

Identify the genotypes of the parents: Father is heterozygous (Aa); Mother is homozygous recessive

Identify the possible gametes: Father can produce two types of sperm (A or a); Mother can produce one type of egg (a)

Apply the Punnett square:

a a A Aa Aa

a aa aa

Calculate genotypic ratio: 1:1 ratio of heterozygous to homozygous recessive (50% Aa and 50% aa)

Calculate phenotypic ratio: 1:1 ratio of HD to normal (50% HD and 50% normal) 7. Explain a monohybrid cross. A monohybrid cross is a single-character cross between two heterozygotes. Mendel discovered that monohybrid cross always result in a 1:2:1 genotypic ratio and a 3:1 phenotypic ratio. Examine the monohybrid cross between yellow peas illustrated below.

8. What is the purpose of a test cross? “An organism that exhibits a dominant trait, such as purple flowers in pea plants, can be either homozygous for the dominant allele or heterozygous. To determine the organism’s genotype, geneticists can perform a testcross. In a testcross, the individual with the unknown genotype is crossed with a homozygous individual expressing the recessive trait (white flowers in this example). By observing the phenotypes of the offspring resulting from this cross, we can deduce the genotype of the purple-flowered parent.” If the parent was homozygous dominant, none of the offspring will have the recessive phenotype. If the parent was heterozygous, there is a 50% chance of offspring having the recessive phenotype. (Text quoted from page 256. See figure below)