The Chromosomal Basis of Inheritance Chapter 15 BCOR 012

Drosophila melanogaster Thomas Hunt Morgan 1866-1945

Morgan’s white-eyed fly wild-type

Wild-type: evidencing the normal phenotype (as opposed to a mutant phenotype). Usually controlled by a dominant allele. Noted as, for example, w+ (this is the allele for normal eye color, red) Mutant: the non-standard phenotype. Usually controlled by a recessive allele. Noted as, for example, w (this is the allele for white eye color)

These observations led Morgan to propose the the eye-color gene is on the X-chromosome; that is to say, it is sex-linked. All the white-eyed flies were male!

Genetic Linkage and Linkage Mapping

Mendel crossed two true-breeding pea lines that differed in seed shape and color. The F1 progeny were all yellow and round. When they were crossed with one another, what were the genotypes and phenotypes among the F2 progeny?

Hypothesis: Independent Assortment Phenotype: yellow, round green, wrinkled X (This is a testcross.) YyRr yyrr Genotype: 1 YyRr : 1 yyrr : 1 yyRr : 1 Yyrr

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So how do you explain the non- parental phenotypes?

The map distance between two genes is the recombination frequency, expressed as a percent.

Clicker Question! D, F, and J are three genes in Drosophila. The recombination frequencies for two of the three genes are shown below. Genes D and F are: A. either located on different chromosomes or located very near each other on the same chromosome. B. located very near to each other on the same chromosome. C. located on different chromosomes. D. either located on different chromosomes or located far from each other on the same chromosome E. located far from each other on the same chromosome.

Hypothesis: Independent Assortment Phenotype: yellow, round green, wrinkled X (This is a testcross.) YyRr yyrr Genotype: Recombinants = 50% N = 575 575 575 575 1 YyRr : 1 yyrr : 1 yyRr : 1 Yyrr

Double crossovers lead to the underestimation of map distances.

The Genetic Basis of Sex Determination

Sex-Linked Traits

Pedigree Showing the Inheritance of Color-Blindness in a Family Characteristics of X-linked genes: Trait shows up more frequently in males than in females Trait is transmitted from mother to son Pedigree Showing the Inheritance of Color-Blindness in a Family

Queen Victoria and her family

X-Inactivation

In order to keep gene dosage balanced, one of the two X chromosomes becomes inactivated early in development. This phenomenon is called X inactivation. Arrows indicate inactivated X chromosomes

Coat color in the tortoiseshell cat results from random inactivation of the X chromosomes.

Clicker Question Q. In which cat, #1 or #2, did X-inactivation happen earlier? A. Cat #1 B. Cat #2 C. They happened at the same relative time in embryonic development. D. It is impossible to tell from the photos. Cat #1 Cat #2

Chromosomal Aberrations Polyploidy: the condition in which the organism possesses three or more complete sets of chromosomes. Aneuploidy: the condition in which certain chromosomes are present in excess or deficit (e.g., trisomy 21).

Aneuploidy arises as a result of nondisjunction at meiosis I or II.

The Down’s syndrome phenotype is the result of nondisjunction of chromosome 21.

In humans, a number of aneuploid conditions result from from non-disjunction of the X chromosomes.

A human cancer, chronic myelogenous leukemia, results from a translocation between chromosomes 9 and 22.