The Chromosomal Basis of Inheritance

25 October, 2002Text Chapter 15

The behavior of chromosomes in meiosis and fertilization explains Mendel’s rules of inheritance.

Genes on different chromosomes assort independently.

Genes and Chromosomes

Morgan’s Mutant

Morgan’s experiments showed that some genes are inherited along with the X chromosome.

Genes that are on the same chromosome are linked. Those on the X chromosome are X-linked.

Remember, the purpose of crossing over is to generate diversity in offspring arising from sexual reproduction.

Crossing over allows linked genes to appear to assort independently.

Crossing over results in recombinant gametes.

Recombinant Offspring

Genes that are far apart are more likely to have a crossing-over event occur between them than genes that are closer together on the chromosome.

Recombination Distance

Knowing the recombination frequencies between several linked genes allows the researcher to construct a genetic map. Note that the maximum genetic distance discernable in a cross is 50% (indistinguishable from independent assortment.) Larger genetic distances are determined by adding.

Mapping

Sex Determination

(carried on the X chromosome) show unique patterns of inheritance. Since the Y chromosome has few genes, recessive sex-linked alleles are not masked in males.

A father with an x-linked trait will transmit the allele to all daughters, but no sons.

A carrier mother will pass the allele to 1/2 of her offspring. Sons will be affected, daughters will be carriers.

Carrier female x affected male produces affected females

Sex-linked genes

X Inactivation

Nondisjunction

Meiotic nondisjunction occurs when homologues or sister chromatids fail to separate in anaphase. This error leads to gametes that have too many or too few chromosomes.

If these gametes are involved in fertilization, the resulting offspring will have an abnormal number of chromosomes (aneuploidy). They may have one (monosomy) or three (triploidy) copies of a chromosome.

This changes the gene dosage of the genes on the affected chromosome

Gene dosage can also be changed by structural alterations within a chromosome, including deletion, duplication, translocation, and inversion.

Alterations of Chromosome Structure

Down Syndrome

Genomic Imprinting

In a plant, leaf color and leaf shape are controlled by two linked genes. Leaves of the wild-type plant are red. A recessive mutation in this gene causes white leaves. Wild-type leaves are pointed, and a recessive mutation in this gene causes them to be smooth. The following crosses were performed: pure breeding white, smooth X pure breeding wild type gives F1: all red, pointed

Now, the next cross: red, pointed X pure breeding white, smooth gives F2:

40 white, smooth 36 red, pointed 10 white, pointed 14 red, smooth

What is the recombination frequency between the gene for color and the gene for leaf shape?

Parental types: 76

Recombinants: 24

Total is 100

Recombinants / Total is 100 / 24 = 0.24

If a man and a non-color blind woman jointly have three sons, two of whom are color blind, which is the most likely explanation?a) The mother is homozygote for color blindness.b) The mother is heterozygote for color blindness. c) The father is color blind. d) None of the above.

A recessive sex-linked lethal mutation is generated in Drosophila. If a female heterozygous for the lethal allele is crossed to a wild-type male, the ratio of males to females expected among the livingprogeny would be:

(a) 0:1 (b) 1:1 (c) 2:1 (d) 1:2

Hemophilia in humans is due to an X-chromosome mutation. What will be the results of mating between a normal (non-carrier) female and a hemophiliac male?

A. half of daughters are normal and half of sons are hemophilic. B. all sons are normal and all daughters are carriers. C. half of sons are normal and half are hemophilic; all daughters are carriers. D. all daughters are normal and all sons are carriers. E. half of daughters are hemophilic and half of daughters are carriers; all sons are normal.