L13 – Sex Linkage PDF

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Notes for Lecture 13 – Sex Linkage...

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Hoosier 1 Genetics Lecture 13 Sex Genetics Part 1 – Autosomal Modes of Inheritance A. Sex Related Modes of Inheritance a. Loci on the autosome where expression is affected by the sex of the individual [Autosome: Chromosome other than a sex chromosome] i. 2 ways it can be expressed: 1. Direct Way a. Once the sex is determined, the expression of whatever genotype you get is “Conditioned” by the individual’s sex (i.e. Environment) 2. Indirect Way a. The locus controlling whatever trait we’re discussing, is on a sex chromosome and is traveling with the genetic code that determines what sex the offspring will be b. Phenotypic Variance: Secondary Sex Characteristics i. Can affect things like: 1. Size 2. Color c. Sex Limited Mode of Inheritance: i. A trait that is expressed in only one sex even though both sexes can carry the alleles 1. Both sexes get the same gene but only one sex expresses it d. Sex Influenced Mode of Inheritance: i. A trait whose expression is conditioned or influenced by the sex of the individual 1. Both sexes will get the same gene, but they will not be expressed the same way B. Sex Limited – Example: Feathering in Chickens a. Two types of feathering that occurs in male & female chickens: i. Simple (hen) Feathering in females  Dominant [HH] ii. Cock Feathering in males  Recessive [CC] b. The feathering of these chickens is very often a sex limited trait c. When we compare the two… Females HH = Hen HC = Hen CC = Hen

Males Hen Hen Cock

Sex Determines Expression

Hoosier 2 C. Sex Influenced – Example: Pattern Baldness a. Classic Male Pattern Baldness is sex influenced i. Bald is dominant in males, recessive in females 1. **REMEMBER: a. Dominant = Only need 1 copy of gene to be expressed b. Recessive = Need to have 2 copies of gene to be expressed Females BB = Bald BN = Non NN= Non

Males Bald Bald Non

Sex Influences Expression Such as here where the trait for balding is Dominant in Males; Recessive in Females

Part 2 – Sex Determination Models A. Sex Determination a. There are 2 main types of chromosomes: i. Sex Chromosome: (Specialized) Chromosome involved in sex determination ii. Autosome: Chromosome other than a sex chromosome B. Modes of Sex Determination a. Protenor Mode: XX/XO System i. XX: Female ii. XO (or just X__): Male iii. Essentially you either get two copies of the same sex chromosome (i.e. you are homozygous) which results in a female 1. Or you get one copy of a sex chromosome (i.e. you are hemizygous) which results in a male Male XO

x

Female XX

X XX

X XX

O XO

XO

X 1 XO Male : 1 XX Female

This is known as the Primary Sex Ratio Primary Sex Ratio: What you expect at fertilization

Hoosier 3 b. Lygaeus Mode [Humans]: 2 Distinct Sex Chromosomes i. XX (or ZZ): Female ii. XY (or ZW): Male iii. Heterogametic Sex : XY or ZW 1. Results in a Male in humans iv. Homogametic Sex: XX or ZZ 1. Results in a Female in humans 2. However, there are some species that do not follow this, in fact they reverse it: a. In many fishes, moths, butterflies, and birds, the individual that receives the homogametic genes is actually the Male v. The ZEDD System: ZZZW 1. The ZZ/ZW represent that these traits are the opposite of what we see in mammals vi. Holandric: [In mammals] Males provide the gamete that determines the sex of the offspring; there is very little genetic information carried on the Y Chromosome 1. Humans pass things Holandricly directly from father to son (so obviously no daughters will receive this genetic information because they don’t receive the Y Chromosome)

Part 3 – Sex Linkage A. Sex Linkage: Mode of inheritance resulting from loci being located on the sex chromosomes a. KEY: Results from Reciprocal crosses will differ Example: Green Male

x

Red Female

Red Male

x

Green Female

b. Where did the Reciprocal Cross come from? Thomas Hunt Morgan i. Used Fruit Flies to determine genetic traits

Hoosier 4 1. Noticed the change in eye color from the usual dominant red eyes to white 2. After an interesting result from a regular Mendelian Cross, Morgan decided to do a Reciprocal Cross a. It was determined, along with many other things that are listed below, that the fruit flies have a Lygaeus Mode of Inheritance i. Remember that in the Lygaeus Mode of Inheritance, males are Hemizygous; meaning that whatever is on the X for the male, they express P1:

Cross A Red x White XR X R X rY

P1: XR X r X R Xr

XR XR Xr

Y XRY

XR Y

Cross B White x Red X rX r XRY

F1:

All Red X R Xr XRY

F1:

1 XRXr  1 Red 1 XrY  1 White

F2:

3 Red [Mix of both Male & Female] 1 White [Only Males]

F2:

1 Red [Male & Female] 1 White [Male & Female]

1 XRXR: Red 1 XRXr: Red 1 XRY: Red 1 XrY: White

1 XRXr  1 Red 1 XrXr  1 White 1 XRY  1 Red 1 XrY  1 White

Hoosier 5...