concept 16 - BIO10008

what are the different mechanisms of sex determination

X and Ys

Distinguish between the transmission of traits carried autosomes and the X-chromosome

autosomal genes affects both males and females, whilst sex-linked traits on the X chromosome affects males more than females since females have two Xs and males have an X and a Y

sex determination mechanism in turtles

temperature dependent

sex determination mechanism in honeybees

ploidy - females are diploid, worker bees are haploids

sex determination mechanism in grasshopper

X system

a male grasshopper has the genotype XO with only one X chromosome, how will this affect the gametes produced during meiosis

normally the chromosome would pair with its homologous partner but since, male grasshoppers only has one X chromosome it will produce gametes with the X chromosome or one without the sex chromosome entirely as there is no homologous pairing.

describe what is meant by a sex-linked trait

sex linked trait is a trait carried on the sex chromosomes only X or Y mostly X

autosomal trait

all offsprings have equal chances of inheriting it

sex linked trait

if the female parent has it, the son or daughter will have it if the father parent has it only the daughter will have it

what is involved in a hybrid cross

involves a cross between two heterozygous parents most likely from self fertilisation

what is a different between a monohybrid cross and a dihybrid cross or a trihybrid cross

the amount of gene locis that are involved

monohybrid - one gene locus

when would you use a test cross

to see what alleles the heterozygous F1 hybrid is passing on by crossing the hybrid with a homozygous recessive parent

when you want to determine the genotype of an individual that shows a dominant phenotype, but whose genotype is unknown.

what is a test cross

A test cross is a genetic cross between:

• An individual with a dominant phenotype but unknown genotype (could be homozygous dominant or heterozygous)

• An individual that is homozygous recessive for the trait in question

benefits of a test cross

to find out the genotype of the unknown dominant expressing phenotypic organism

(to determine the genotype of an organism that expresses the dominant phenotype)

what are the phenotypes of an individual who is homozygous for a recessive lethal allele

death

what outcome of a cross would lead you to suspect that a recessive lethal allele was involved

if the offsprings were made from two heterozygous parents, then the ratio appeared to be 2:1 is because the fourth offspring died from the homozygous recessive lethal alleles

reciprocal crosses

crosses where the female parent is affected with the trait and another one where the male parent is affected with the trait

utilities of all genetic crosses

Cross Type	Reveals	Ratio (Typical)
Monohybrid	Dominance, segregation	3:1 or 1:2:1
Dihybrid	Independent assortment	9:3:3:1
Test Cross	Unknown genotype, linkage	Varies
Sex-linked	X-linked inheritance patterns	Gender-specific ratios
Linked Genes	Gene linkage, recombination	Deviation from 9:3:3:1
Incomplete/Codominance	Blended or dual traits in heterozygotes	1:2:1
Polygenic	Traits influenced by multiple genes	Continuous variation

what is the difference between a family tree and a pedigree

family tree is showing the familial connections but pedigrees show family connections and inheritance of traits

what information might be useful for a genetic counsellor to know when diagramming a pedigree

suspected carriers, individuals that were affected and individuals’ genders

open circle

unaffected female

shaded circle

affected female

open square

unaffected male

why is it that a dominant trait will be observed in every generation of a pedihree but a recessive trait will likely skip generations

recessive traits need to be homozygous to be expressed, if there is a dominant trait it will be expressed regardless if it is heterozygous or homozygous. (dominant needs only one copy for expression)

males are observed to have 29 chromosomes whereas females have 30 explain this observation and the differences in the gametes produced by each of the sexes

why are x-linked disorders more common in males than females