Linked genes & Sex-linked genes

Genes & Chromosomes

• Occur in pairs in 2n nuclei
• Segregate at meiosis, both exhibit independent assortment
• Unite at fertilization

We will examine some relationships between chromosomes & genes

• A cell contains a few chromosomes
• Example: A human nucleus contains 23 different chromosomes
• A cell contains many thousands of genes
• Therefore, each chromosome contains hundreds or thousands of genes

Linked genes

• Genes close together on same chromosome called linked genes
• Linked genes do not exhibit independent assortment

Unlinked genes - review

• P: YYRR (yellow round) x yyrr (green wrinkled)

• F₁: YyRr (yellow round)
• Test cross the F₁:

• How did the recombinations come about?
• Independent assortment - Y had an equal chance of going with R or r

Linked genes

• Will look at crosses with fruit flies - Drosophila melanogaster
• Used from early 1900s - ideal for planned breeding experiments
• short life cycle (10-14 d)
• easy to handle,
• large nos. of crosses
• produces many offspring
• has many traits to follow

Linked genes

• Will examine two genes in fruit fly:
• gene for body color - B=gray, b=black
• gene for wing length - S=long, s = short (vestigial)
• Both genes are close together on same chromosome
• Don’t exhibit independent assortment

Linked genes

• P: BBSS (gray, long) x bbss (black, short)
• BS bs gamete genotypes
• F1 BbSs (gray long)
• Test cross: BbSs x bbss
• BS bs bs gamete genotypes
• (B&S are linked, b&s are linked)

Actual results

• BbSs (Gray long) x bbss (black short)
  

• 17% recombinants

Explanation

• black, short alleles on one chromosome
• gray, long alleles on homologue
• Alleles could be exchanged between homologous chromosomes by crossing over
• Frequency of crossing over between these alleles was 17%
• If these genes exhibited independent assortment then recombination frequency would be 50%

Genetic symbols for fruit flies

• For a given character - the gene takes its symbol from the first mutant discovered
• For black bodies - b
• For short wings - vg (for vestigial)
• The normal phenotype is called wild type & is indicated with a superscript +
• For wild type - gray bodies - b⁺
• For wild type - normal wings - vg⁺
• If mutant is dominant, then uppercase letter - curly wings=Cy, wild type=Cy⁺

Previous cross with new symbols

• P: b⁺b⁺ vg⁺vg⁺ (gray long) x b b vg vg (black vestigial)
• b⁺vg⁺ b vg gamete genotypes.
• F₁ b⁺b vg⁺vg (gray, long)
• Test cross: b⁺b vg⁺vg x b b vg vg

Test cross - 17% recombinants, See Fig. 15.5 Possible results

• If independent assortment:
• 50% parental - gray, long & black, vestigial
• 50% recombinant-gray, vestigial & black, long
• If linked & no crossing over (never)
• 100% parental - gray, long & black, vestigial
• If linked & crossing over occurs (always)
• Frequency of recombinants dependent on frequency of crossing over between genes, can only be determined experimentally

Crossing over , See Fig. 15.6

• Responsible for recombination between linked genes - genes on the same chromosome
• The closer the genes are to one another, the less frequently they recombine
• Crossing over frequency is proportional to distance between genes

Crossing over separates linked genes Using recombination frequencies to map genes

• Genes are on a linear chromosome
• The closer 2 genes are to each other, the lower the frequency of crossing over
• By determining recombination frequency between linked genes can determine relative distance between them
• "map unit" = 1% recombination frequency

Mapping genes, See Fig. 15.7 & 15.8

• If b & vg are 17 map units, b & cn are 9 map units, vg & cn are 9.5 map units
• What is sequence of genes on chromosome?
• Correct sequence is b-cn-vg

Mapping genes

• Crossing over occurs more commonly in some regions than others
• So recombination frequencies are not fixed lengths of a chromosome
• Linkage map gives sequence of genes but not precise locations on chromosome

Example

• Recombination frequencies:
• X& Y is 15%
• X & Z is 7%
• Y & Z is 22%
• What is sequence of genes?
• X 15 Y
• X 7 Z or Z 7 X
• Z 7 X 15 Y
• Correct sequence is ZXY

Sex chromosomes

• In animals, males & females differ with respect to one pair of chromosomes - sex chromosomes
• all other chromosomes - autosomes
• Sex chromosomes in humans & others:
• females have 2 X chromosomes, female genotype is XX
• males have 1 X & 1 Y chromosome, male genotype is XY

Sex chromosomes

• X chromosome - carries many genes, called sex-linked genes
• Y chromosome - small, only carries a few genes that are responsible for sex determination, such as Sry (sex determining region of Y)
• Other animals have different system for sex determination

Sex chromosomes

• Humans have 22 pairs of autosomes & 1 pair of sex chromosomes
• P: XX x XY
• X X Y gamete genotypes
• F₁ 50% XX, 50% XY
• Which parent determines sex of child?

Back to fruit flies

• Drosophila is 2n = 8, has 3 pairs of autosomes, 1 pair of sex chromosomes
• Has XY system of sex determination

Early experiments with flies, See Fig. 15.3 & 15.4

• One of earliest mutant phenotypes was white eye color - w
• Wild type is red - w⁺

Crosses with white eyed flies

• red eyed female x white eyed male
• F₁: all red eyes Conclude:
• red eyes dominant
• But white eyed female x red eyed male
• F₁ red eyed female & white eyed male
• Explanation?
• Gene for eye color carried on X chromosome, males only have 1

Sex-linked traits

• Carried on X chromosome, not on Y
• Males obtain X from mother, so inherit sex linked traits from mother
• Eye color of fruitflies:
• w⁺w⁺ (red eye female ) x w Y (white eye male )

F1: w⁺w & w⁺Y
Both female & male have red eyes

Sex-linked traits

• Reverse cross:
• w w (white eye female) x w⁺Y (red eye male )

F₁ w⁺w (red eyed female ) w Y (white eyed male )
females have red eyes, but males have white eyes

Sex-linked cross, see Fig. 15.4

• red eye female x white eye male
• F₁ all red eye
• F₂ 100% red eye female, males -50% red eye & 50% white eye

Sex-linked crosses

• In humans, colorblindness is a sex-linked trait
• C - normal vision
• c - colorblind
• Genotypes Phenotypes
• CC normal vision, woman
• Cc normal, but carrier
• cc colorblind woman
• CY normal vision, man
• cY colorblind man

Problem

• Woman with normal vision but a carrier marries a man with normal vision. What proportion of their boys will be colorblind? of their girls?
• Cc x CY

50% of boys are colorblind
All girls have normal vision

Sex-linked traits

• Males contain only 1 allele for sex-linked traits - on X chromosome
• Males inherit sex-linked traits from mother
• Females inherit sex-linked traits from each parent

X inactivation in females

• Females have 2 X chromosomes
• In each cell, one X is inactivated, becomes Barr body
• Females have only 1 active X
• Either can be inactivated during embryonic development
• Females contain some cells with X chromosome from mother and some with X chromosome from father inactivated

Example: Tortoiseshell cat color,See Fig. 15.11

• Sex linked gene for coat color - one allele for black, another for orange
• Female that is heterozygous - some cells produce orange, others black (males will be orange or black)