Unit 5 - Sex Linkage and Pedagrees Flashcards
Autosomal Inheritance
Refers to the transmission of genes carried on autosomes, the 22 non-sex chromosomes
Autosomal dominant inheritance (4)
- males and females have approx the same frequency of trait
- Each individual with trait must also have one parent with the trait
- If neither parent has the trait, none of their children will have it
- if the trait is rare, (15% ish) then those with the trait are likely heterozygous
Autosomal Recessive inheritance (4)
- males and females have approx the same frequency of trait
- Those with trait are often born from those without it (heterozygotes)
- If both parents have trait, then all children will have it
- If only one parent has the trait, a child can only have it if the other parent is heterozygous
Pedegrees
Square = male
circle = female
black = has trait
white = no trait
line through = dead
Wild type
the phenotype most common in the population
Sex-linked inheritance
transmission of genes on sex chromosomes. X and Y
Sex determination
the genetic and biological processes that produce the male and female characteristics of a species
Sex determination in drosophila (Fruit flies) (4)
*Females have two X chromosomes and males have one X chromosomes
*So males have XY (normal), XYY or XO
*Females are XX(Normal), or XXY
*XXX flies are very rare and YO flies dont exist
X/A ratio (2)
*The X / Autosome ratio determines gender for some species based on the number of X chromosomes to sets of autosomes
*Males typically have a 0.5 ratio and females have a 1.0. So in fruit flies, the female has 2 X’s and 2 sets of autosomes so 2/2 is 1. In males they have one X and 2 sets of autosomes so 1/2 =0.5
Mammalian Sex determination (3)
*Depends on the presence or albescence of a single gene (SRY) found on the Y chromosome which produces a transcription factor needed for male-specific gene expression
* In mammals, females can be XX (Normal), XO or XXX.
*Males can be XY (Normal), XXY or XY
SRY (3)
*all mammal embryos have clusters of tissue called undifferentiated gonads, which develop into ovaries or testes
*Expression of SRY initiates testicular development of the undifferentiated gonads
*The absence of SRY expression allows the default state, female, to develop.
Z/W system (2)
*used by many birds, reptiles, fish, butterflies and moths
*Females have two different sex chromosomes (ZW) and males have two of the same (ZZ)
Hemizygous
refers to only having one copy of a gene rather than the normal two. For example in male mammals, they have one copy of X and one copy of Y, so they are hemizygous for those genes.
X-Linked recessive inheritance (3)
*Females are homozygous for the recessive allele
*Males are hemizygous for it and display the recessive phenotype as there is no second X to mask the trait
*Expression of trait is much more common in males than in females
X-linked dominant Inheritance (3)
*Females are heterozygous or homozygous dominant (assume hetero)
*males are hemizygous for the dominant allele
*both express the dominant phenotype
Y-linked inheritance
exclusively patrilineal (passed from father to son)
X-linked recessive pattern (5)
*skips generations when male -> female -> male as the 2nd X for females masks the recessive allele
*Much more frequent in men than women
*A male with the trait mating with a homozygous dominant female will produce all offspring with the dominant phenotype, where all female offspring are carriers
*Mating with recessive male and carrier females will produce about half dominant and half recessive offspring
*mating of dominant male with recessive female results in all carrier female offspring and all recessive male offspring
X-linked dominant pattern (2)
*Heterozygous females mated to recessive males transmit the dominant allele to half their offspring regardless of sex
*Dominant males mated to recessive females pass the dominant trait to all daughters and none of their sons
Y-linked pattern
*exclusively male-to-male as genes on the Y chromosome do not have a copy on the X chromosome to mask them
Dosage compensation (2)
*In any organism with sex chromosomes, there is an imbalance with the number of genes on the sex chromosomes (X is bigger and has more than Y)
*Any mechanism that compensates for this is called dosage compensation
4 Types of Dosage compensation
- In fruit flies, expression of X-linked genes in males is doubles compared to females
- In roundworms, expression of X-linked genes is halved in females compared to males
- in marsupial mammals, the paternal X chromosome in females in inactivated
- In placental mammals, one random X chromosome in females is inactivated
Random X-inactivation hypothesis (4)
- as a form of dosage compensation, one random X chromosome is inactivated
*once this occurs it is locked in for all decedents of said cell
*Female mammals are mosaics of two populations of cells; one expressing the maternal X and the other expressing the paternal X
*expression for maternal and paternal X chromosomes is approx equal
Homogametic
produce two copies of a sex chromosome (XX or ZZ)
Heterogametic
produce two different sex chromosomes (XY or ZW)
