Home Biology 3020 Introduction to Evolution

Winter 2008
 

Beyond Mendel

I. Genetics is more complicated than Mendel imagined

A. Dominance

1. Mendel demonstrate complete dominance, i.e. dominant gene totally masks the recessive

2. Incomplete dominance - offspring show traits intermediate between two parental phenotypes

a. Red and white-flowered four o'clocks produce pink-flowered offspring

b. Does not support a blending theory; parental phenotypes reappear in F2 generation

3. Codominance - both alleles of a gene are expressed

a. E.g. both A and B alleles are expressed in human blood type AB

B. Multiple Alleles - may be more than two alleles for one locus, but each individual inherits only two alleles

1. E.g. ABO system of human blood type involves three alleles

2. As a result, there are four possible phenotypes or blood types: A, B, AB, and O

C. Pleiotropy -  a single gene exerts an effect on many aspects of an individual's phenotype

1. E.g. sickle cell anemia is caused by a single mutation in the hemoglobin gene. It produces a number of effects

D. Environment affects the phenotype

1. Both genotype and environment affect phenotype; relative importance of both influences vary

2. Aquatic environment influences the phenotype of water buttercup

3. Temperature can affect phenotype e. g. Siamese cats, Himalayan rabbits 

E. Polygenic inheritance - a trait is controlled by several allelic pairs at different loci

1. Polygenic traits are subject to environmental effects that cause intermediate phenotypes; so they produce continuous variations whose frequency distribution forms a normal (bell-shaped) curve, human height is a good example

II. 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 (production of sex cells) 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

b. hemophilia spread throughout the royal families of Europe, probably after a mutation in Queen Victoria's father

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, at about seven 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

III. Additional reading

A. DNA from the beginning