The Discovery of Chromosomesand

Sex-Linked Traits

Outcomes: 1. Compare the pattern of inheritance

produced by genes on the sex chromosomes to that produced by genes on autosomes, as investigated by Morgan.

2. Predict, quantitatively, the probability of inheritance of sex-linked traits.

A Short History of Genetic Understanding● Middle ages: dissect corpses to discover details of internal

anatomy ● Genetics: advancement upon creation of the light

microscope- discovery of nucleus 1831 -1865: Mendel published papers on garden peas, it was known that egg and sperm create a zygote, “factors” from each parent blended to form offspring

● New microscope advancements - cytology (studying cell formation & structure)

● 1882: Fleming discovered formation of threads in nucleus during cell division

● 1882: van Benden noticed sperm and egg cells of roundworm had 2 chromosomes, fertilized eggs had 4

● 1887: Weisman suggested special division occurred in sex cells (we now call this meiosis!)

Chromosomal Theory● 1902: Sutton & Boveri

-studied Dresophila ● strands of genetic info come in pairs and

segregate during meiosis - form new pairs during fertilization

● “Factors” or genes made up of alleles, one allele from each parent- union of alleles and formation of new combinations explained Mendel’s theories

● Sutton & Boveri reasoned that expression of traits is not only dependent on male vs. female sex cells - structures within determine heredity, “factors” located on chromosomes

● Deduced that humans have 44 autosomes, 2 sex chromosomes, but thousands of different traits (‘factors’)- each chromosome carries genes

● Genes on same chromosome are LINKED GENES

Chromosomal Theory: Summary● Chromosomes carry genes, the units of heredity ● Paired chromosomes segregate during meiosis. Each

sex cell or gamete has half the number of chromosomes found in the somatic cells. - This explains why each gamete has only one of each of the paired alleles

Sex-linked Traits● Thomas Hunt Morgan ● Used Drosophila to study

principles of inheritance- reproduce quickly - large numbers of offspring, small size- female easily distinguished from males

● Eye colour: crossed white-eyed male with red-eyed female- all F1 offspring red eyed

● Mated two hybrids from F1 - F2: ¾ red, ¼ white- females all red, white only found in males - of the males, ½ red, ½ white

Sex-linked traits

● Sex chromosomes only partially homologous - Morgan concluded they contain different genes

● Females have two X chromosomes ● Males have one X and one Y chromosome ● Morgan concluded: Y chromosome does not carry gene

for eye colour- we now know it is on the X chromosome!

● Traits determined by genes on sex chromosomes are called sex-linked traits

Morgan’s Experiments and Sex-Linked Traits● Consider the original problem:

- pure-breeding red-eyed female (XRXR)- pure-breeding white-eyed male (XrY)- XR indicates that the allele for red eye is dominant and is located on the X chromosome- no symbol for eye colour on the Y because it does not contain an allele for the trait.

Sex-linked Traits● Alleles on X-chromosome determine

trait- ex) colourblindness, hemophilia, hereditary myopia, night-blindness

● Mothers pass on trait to their SONS ● Not impossible for daughters but much

less likely ● Males cannot be homozygous for X-

linked gene- only have one X chromosome

● Recessive lethal X-linked disorders in humans occur more frequently in males - ex) infantile spinal muscular atrophy

Sex-linked Traits Example ● In humans, the recessive allele that causes a form of

red-green colour-blindness (c) is found on the X-chromosome. a) Identify genotypes and phenotypes of the F1 generation from a colour-blind father and a mother who is homozygous for colour vision.

● B) Identify the F1 generation from a father who has colour vision and a mother who is heterozygous for colour vision

● C) Use a Punnett square to identify parents that could produce a daughter who is colour-blind

Practice1. A woman who has normal vision and the

heterozygous genotype XCXc marries a man who is colour blind (XcY). What is the expected ratio of genotypes and phenotypes among their children?

2. In a species of dog, a mutant gene that causes deafness is found on the Y chromosome. Draw a punnett square to show the outcomes of a cross between a) A male dog whose father is deaf and a female dog

whose father is not deaf.

b) A female dog whose father is deaf and a male dog whose father is not deaf.

3. Read the section on Barr Bodies (begins on pg 603). a) Define Barr Bodies

b) Why does the formation of Barr bodies mean that human females only have one functioning X chromosome in each somatic cell?