Chromosomes and genes

Biology 1010

Fall 2009

CHROMOSOMES

I. Chromosomal theory of inheritance

A. Boveri and Sutton (1902) proposed that genes are on the chromosomes

1. Genetic material must be in the nucleus because:

a. Nuclei fuse to form the zygote

b. In both plants and animals sperm cells consist of mostly just a nucleus

c. Sperm and egg make equal contributions to heredity of offspring

2. Genes must be on the chromosomes because the behavior of chromosomes during meiosis corresponds to the distribution of genes in the progeny

a. Both chromosomes and factors (alleles) are paired in diploid cells

b. Chromosomes and alleles of each pair separate during meiosis so gametes have one-half the total number

c. Chromosomes and alleles of each pair separate independently so gametes contain all possible combinations

d. Fertilization restores diploid chromosome number and paired condition for alleles in zygote

B. Sex chromosomes determine gender

1. In most animal species, chromosomes can be categorized as two types:

a. Autosomes = nonsex chromosomes that are the same number and kind between sexes

b. Sex chromosomes - differ in number and kind between males and females

2. Sex chromosomes in the human female are XX; those of the male are XY

3. Males produce X-containing and Y-containing gametes; males determine the sex of offspring

4. Besides genes that determine sex, sex chromosomes carry genes for traits unrelated to sex

5. X-linked gene - any gene located on X chromosome; used to describe genes on X chromosome that do not control a sexual feature and that are missing on the Y chromosome

a. Common sex linked traits include baldness, hemophilia and color blindness

C. Human sex

1. Y chromosome carries a master gene SRY (Sex determining Region of the Y chromosome)

2. Humans females have XX while males have XY

3. If an embryo has two X’s then the female reproductive organs, ovaries, will develop, produce female hormones and the individual will become a female

4. If an embryo has an X and Y, after a few weeks the SRY master gene on the Y is turned on. It causes the formation of the male reproductive organs, testes, which produce male hormones and the individual becomes a male

II. Chromosomes undergo mutations

A. Mutations - changes in chromosomes or genes that pass to offspring

1. Increase the amount of variation among offspring

2. Chromosomal mutations include changes in chromosome number and structure

B. Polyploidy - offspring end up with more than two complete sets of chromosomes

a. Rare in animals but a major evolutionary mechanism in plants

b. Hybridization in plants can result in doubled number of chromosomes; an even number of chromosomes can undergo synapsis during meiosis; successful polyploidy results in a new species

species			hybrid	chromosomes double	polyploid
AA	BB	---------->	AB	----------------------------->	AABB
(24)	(30)		(27) sterile		(54) fertile

c. Down syndrome is most common trisomy among humans; 3 copies of chromosome 21

C. Changing chromosomal structure

1. Environmental factors including radiation, chemicals, and viruses, can cause chromosomes to break; if the broken ends do not rejoin in the same pattern, this causes a change in chromosomal structure