Lec. 22: Genes and Chromosomes III: Alterations of chromosomes

Question 1

Thomas Hunt Morgan

Answer 1

• Concluded that fly’s eye color is linked it it’s sex

Question 2

Sex-linked Genes

Answer 2

Genes located on a sex chromosome exhibit unique inheritance patterns

Question 3

Gene Linkage
(Give Example)

Answer 3

Genes located on the same chromosome that tend to be inherited together

Body color and wing size are usually inherited together because their genes are on the same chromosome

Question 4

What mechanism breaks linkage between genes on the same chromosome?

Answer 4

CROSSING OVER breaks linkage between genes on the same chromosome

(Not all alleles from a chromsome will be passed on to a gamete)

Question 5

What is recombinant frequency and what is it related to?

Answer 5

Recombinant frequency is the percentage of recombinant offspring
It is related to the distance between the genes

Recombinant gametes are produced in Prophase I

Question 6

Linkage maps

Answer 6

Shows the relative locations of genes and recombinant frequencies along a chromosome

Question 7

How do sex linked genes give rise to unique patterns of inheritance?

Answer 7

• ## The sex chromosomes, especially X, have genes for many characters unrelated to sex

Question 8

How are genes on the X expressed at equal levels in males and females?

Answer 8

By shutting off one X chromosome in females

Question 9

Barr Body

Answer 9

Only one X chromosome out of two is active so males and females have the same one copy of genes on the X chromosome
- During female development, one X chromosome per cell condenses into a compact object called a Barr Body which inactivates MOST of its genes

Reactivated when woman is producing eggs

Question 10

What can Barr Body Dependent Inheritance Patterns lead to?

Answer 10

• If a female is heterozygous for a sex linked trait, half her cells will express one allele and the other half will express the other allele (Hydrohidrotic ectodermal dysplasia is lack of sweat glands in some areas)
  -Tortoise shell cats have orange and black patterns due to patches of cells expressing an orange allele while others have non orange alleles

Cell has active Black allele on 1 X chromosome on black part of cat and inactivate orange

Question 11

Aneuploidy

Answer 11

An abnormal number of chromosomes

ie XXY on male calico cat

Question 12

Examples of Aneuploidy of sex chromosomes in humans

Answer 12

XXY, males, 1/1000- Klinefelter syndrome
XYY, males,1/1000, Jacob’s syndrome
XXX, in females, 1/1000, Triple X syndrome
X0, in females, 1/2500, Turner’s Syndrome

Question 13

Genomic Imprinting

Answer 13

Occurs during formation of gametes and results in the silencing of one allele of certain genes
- Either mother or fathers gene is silenced due to DNA methylation

NOT SEX LINKAGE

Question 14

What are alterations of chromosome structure

Answer 14

Deletion- A deletion of a chromosomal segment
Duplication-A duplication repeats a chromosomal segment (results in extra genes)
Inversion-An inversion reverses a segment within a chromosome (segment B,C,D becomes segment D,C,B)
Translocation-A translocation moves a segment from one chromosome to a nonhomologus chromosome (Chromosomes swap ends)

Question 15

What is one result of chromosome translocation?

Answer 15

Translocation of Chromosome 9 and 22 results in the Philadelphia Chromosome. Where the two ends meet generates a novel fusion protein (BCR:ABL) which is a kinase that controls cell-cycle. This results in chronic myelogenous leukemia (CANCER)