Transcript 15.2 Sex linked genes exhibit unique patterns of - TJ

Chapter 15 The Chromosomal Basis of Inheritance
15.1 Mendelian inheritance has its physical basis in the behavior of
chromosomes
I. Chromosome theory of inheritance
A. Chromosomes undergo segregation & independent assortment
1. Where are alleles located & where did they come from?
II. Morgan’s experimental evidence: Scientific inquiry
A. Morgan’s choice of experimental organism
1. The fruit fly: Drosophila melanogaster
a. Advantages
1. Reproduce quickly & in large #s
2. Only have 4 chromosomes
a. 3 Autosomes & 1 sex (XX or XY)
b. Terminology
1. Wild type
a. Phenotype most commonly observed in natural pop.
b. + designation w+ = red eyes
2. Mutant phenotypes
a. Phenotypes that are alternatives to wild type
1. Mutation of wild type
b. No designation w = white eyes
B. Correlating behavior of a gene’s alleles with behavior of a
chromosome pair
1. Morgan’s experiment
a. Red eyed female (w+) X white eyed male (w)
1. F1 all red eyed
b. F1 Red female X red male
1. 3 red 1 white
a. Only males had white
1. Eye color linked to sex chromosome
a. Supports chromosome theory of inheritance
15.2 Sex linked genes exhibit unique patterns of inheritance
I. The chromosomal basis of sex
A. XX or XY for most
1. Humans
a. 2 month embryo sex determination begins
1. SRY Sex determining Region of Y
a. No SRY= female
II. Inheritance of sex-linked genes
A. Males pass only to daughters
B. Females pass to both daughters and sons
C. Examples
1. Male patter baldness
2. Color blindness
3. Duchenne Muscular Dystrophy
4. Hemophilia
III. X inactivation in female mammals
A. Barr body
1. In females 1 X chromosome becomes inactive & condenses
into a Barr body
a. Prevents over production of proteins
2. Which X becomes inactive?
a. Random & independent in each embryonic cell
1. Results in 1/2 the cells with maternal X active and 1/2
the cells with paternal X active
a. Will be passed on during mitosis
3. Affects
a. 1/2 the cells will express paternal
genes & 1/2 will express
maternal genes
1. Male patter baldness
2. Tortoiseshell cats
4. Reactivated in cells giving rise to eggs
B. How
1. DNA modification DNA methylation
a. Methyl groups attached to nitrogenous bases
1. — CH3
2. XIST gene
a. X-Inactive Specific Transcript
b. Active only on Inactive X chromosome
15.3 Linked Genes
I. Intro
A. Mendelian inheritance
1. Law of segregation
2. Law of independent assortment
RRNN x rrnn
RrNn x rrnn
B. Chromosome theory of inheritance
1. Supported by Morgan
RrNn x RrNn
C. Morgan’s experiments
1. After hundreds of crosses discovered many mutants
a. Performed 2 trait crosses & test crosses to study
b. Conclusion: did not always get expected ratio
D. Inheritance reality
1. We do not always see the expected ratio for offspring
a. Why?
b. However
1. Morgan did observe a small pattern
a. A greater proportion of individuals exhibited parental
phenotypes than would be expected given
independent assortment
b. Observed that certain traits tend to be inherited
together
1. Concluded that some traits tend to be inherited
together because they are located on the same
chromosomes linked traits
II. How linkage affects inheritance
A. Morgan’s experiment (part deux)
1. Reality check
a. Most offspring have parental phenotypes
1. Linked genes
b. However, once again not exactly what was expected
1. Some individuals have nonparental phenotypes
2. Some mechanism must be breaking the linked genes
apart
a. What?
III. Genetic recombination & linkage
A. Recombination of unlinked genes: Independent assortment of
chromosomes
1. Results from random orientation of homologous
chromosomes in metaphase I
2. Expect to see a 50% frequency of recombination
a. A 1 : 1 : 1 : 1 ratio
B. Recombination of linked genes: crossing over
1. Morgan’s results
a. 83% parental type; 17% recombinant
b. Results from crossing over during prophase I
IV. Mapping the distance between genes using recombination data
A. Genetic map
1. An ordered list of genetic loci along a particular chromosome
a. Shows relative locations of genes along a chromosome
1. More precisely the linear order but not physical
location
b. Allows us to better predict which genes will be linked &
which will not
B. The process
1. Based on the direct relationship between the frequency of
crossing over (% of recombination) & distance between
alleles
a. % = map units
2. Find the greatest # of map units between 2 loci & place on
map
a. This will be either end of map
3. Place the rest in the map at appropriate distances
4. Examples
a. Example 1
1. Genes A & B are 8 map units apart
2. Genes B & C are 10 map units apart
3. Genes A & C are 2 map units apart
What is the order of these genes?
b. Example 2
1. The distance between Black body & purple eye alleles
is 6 map units
2. The distance between purple eye & vestigial wing
alleles is 12.5 map units
3. The distance between black body & vestigial wing
alleles is 18.5 map units
Where are the alleles located on the chromosome?
c. Example 3
1. A-B = 10 map units
2. A-C = 8 map units
3. A-D = 18 map units
4. C-D = 10 map units
C. The point of genetic maps
1. The closer 2 genes are the greater the chance of linkage
between them
15.4 Alterations of chromosome number or structure causes some
genetic disorders
15.5 Some inheritance patterns are exceptions to the standard
chromosome theory
I. Genomic imprinting
A. Variations on phenotype depending on maternal or paternal
inheritance
B. Occurs during gamete formation
1. One allele of a gene is silenced
a. Results in zygote expressing only 1 of the alleles
2. Imprinting different in sperm & egg
C. Transmitted to all body cells