Topic 4 - Sex linkages and Pedigrees

Autosomes

equally represented in both sexes

Sex chromsomes

differently represent in the sexes, hemizygous

homogametic

same sex chromosomes (XX female or ZZ male)

heterogametic

different sex chromosomes (XY males or ZW females)

Pseudoautosomal regions 1 and 2

terminal ends, homologous regions between different sex chromsomes, allows for pairing up during meiosis,

hemizygous

differential regions, different genes

sex linkage

genes in differential genes

X-linked inheritance

inheritance of genes on X chromosomes, probability of transmission not the same fo each sex

Y linked inheritance

inheritance of genes on Y chrosomes

Drosophila melanogaster (fuit fly)

model organsims, stuidy of evolution from environmental context, easy to rear and cross, easy to aquire mutants, short life spans, multiple offspring (100), small genome (2N=8)

Thomas Hunt morgan

discovery of sex linked traits, studied drosophila melanogaster

wildtype

phenotype most common in population, denoted with +

mutants

phenotypes different from wildtype, denoted with m

carriers

carry both domiant and recessive alleles for trait (heterozygous)

indication of sex linked inheritance

male and females have different phenotypic ratio, reciprocal crosses give different results, criss cross inheritance

nondisjunction

failed chromosome separation

X linked recessive

recessive allele on X chromosome, phenotype expressed in homozygous recessive females and hemizygous males

X linked dominant

dominat allele on X chromosome, expressed in homo/heterozygous females and hemizygous males

Autosomal recessive

recessive allele on autosomes

autosomal dominant

dominant allele in autosomes

Y linked inheritance

solely within male lineages (patrilineal), generally male specific traits

Mitochondrial inheritance

mitochondria DNA (mtDNA), matrilineal, all offspring of effect mother would have

complete penetrance

known genotype = known phenotype, genotype always produces same phenotype

sex limited traits

autosomal, alleles carried in both sexes but different phenotypes

Sex influenced traits

autosomal, inheritance patterns vary between sexes even if same genotype, influenced by hormone levels, behave like autosomal but only in one sex

chromosomal sex

presence of sex chromosomes, determined at fertilization

phenotypic sex

internal and external morphology, determined by gene expression, canbe modified

sex determination in Drosophila (nondisjunction)

Xy/XYY/X0 = males, XX/XXY = females, XXX/Y0 = lethal

Drosophilia sex ratio (X:A)

>1 female, <0.5 male, 0.5-1 intersex

sex determination in mammals

XY/XXY/XYY = male (any Y), XX/X0/XXX females

Sex determining region on Y (SRY)

why sex deterimantion in mammals depends on presence of Y chromosomes, expression leads to development of male structures

Wolffian duct

develops into male sexual and reproductive structures, SRY expression initiates

Mullerian ducts

develops into female sexual and reproductive structures, absence of SRY allows

Sex determination in Birds/reptile/fish/moth/butterflies

ZW = females, ZZ = males

ZW system

female heterogametic/hemizygous (ZW), males homogametic (ZZ), Zlinked genes behave like X linked genes, W linked are matrilineal (only in females)

sex determination in Roundworms/C.elegans/nematodes (X:A ratio)

XX= hermaphrodite (self fertilizes), nondisjuction results X0 = males

males in Roundworms/C.elegans/nematodes

produce super sperm (out competes hermaphrodites), increase gentic variation, offspring are 1 XX: 1X0

Sex determination in Montremes

5 pairs of sex chromsomes, 5 set of XY = males, 5 set XX = females

sex determination turtles

environmental factor, <28C = males, >32C = females, 28-32C = both

sex determination in some fish (clown fish)

change sex throughout life , depends in tempertature

sex determination in plants

90% bisexual (hermaphroditic), 10 % separe plants/ hermaphrodites and males/ hermaphrodites and females

Dosage compensation 

balance the expression of X (or Z) linked genes

Dosage compensation in flies

expreesion of X linked genes doubled in males

Dosage compensation  in roundworms

expression of X linked gens in hermaphrodites halved

Dosage compensation in marsupial mammals

paternal X chromosmes inactivated

Dosage compensation in placental mammal

one X chromsomes randomly inactivated in each somatic cell (Barr bodys), early on embryonic development (~2weeks in humans)

Lyon hypothesis

½ somatic cell inactivate maternal X chromosomes, ½ somatic inactivate paternal X chromsome, inactivated X chromsomes becomes Barr body (transcription regualtion)

X inactivation

epigentic regulation, formation heterochromatic in one silence transcription

X-inactive specific transcript (Xist)

17kb long, noncoding RNA, antisense Tsix perfeoms silencing

Steps of X inactivation

X chromosmes temporarily pair, repress transcription of Tsix from onalleles (establish inactivated), transcription of Tsix on Xa blocks activation on Xist (establish active), Xist IncENA spread along Xi inducing silencing (by YY1 protein and recruits PRC2 which catalyzes heterochromatin modifications)

X inactivation in cats

female heterozygou cats have allele for black fur and another for orange, inactivation result in fur patches of different colours, only males if XXY (infertile)

Pedigree analysis

study hereditary transmission, graphically mapped, follows genetic disorders, can look at several traits, used to predict genetic disorder risk

propsitus

person geneticists is examing

recessive alleles in pedigrees

skip generation, if both parents are homozygous all children are, male/female affected equally

dominat alleles in pedigrees

does not skip generations. male/female affected equally, parent ethier sex can transmit, if neither parent has none of their offspring will, can produce offspirng without if heterozygous

Autosomal traits in pedigrees

evenly occur in both sexes

X linked recessive in pedigrees

more common in males, appear to skip generation, affected female passes to all her sons, carrier females pass to ½ her offspring, affected male pass to all his daughter but none of his sons

X linked dominat in pedigrees

more common in females, all daughter of affect male affected, doesn’t skip generation

mitochondrial in pedigrees

materllineal (only females pass on)

Y linked in pedigrees

only affects males, all sons of affected man affected