3.7.1 Inheritance

genotype

the genetic constitution of an organism

phenotype

the expression of an organism’s genetic constitution combined with its interactions with the environment

dominant allele

an allele whose characteristic will always appear in phenotype

recessive allele

an allele whose characteristic will only appear in phenotype if there are 2 present + no dominant allele is present

homozygous + heterozygous

-homo = 2 identical alleles

-hetero = 2 different alleles

monohybrid inheritance

where one phenotype characteristic is controlled by a single gene

diagrams to display inheritance

1) genetic diagrams

2) punnet squares

codominant alleles

2 dominant alleles that are both expressed in phenotype

sex linkage

where an allele is located on one of the sex chromosomes meaning its expression depends on the sex of the individual

X and Y chromosomes

-males = XY, females = XX

-the Y chromosome is shorter than the X chromosome and carries fewer genes so most genes are carried on X chromosome

why are males more likely to get recessive phenotype

-as males have one X chromosome, they only get 1 copy of the allele so will express it even if recessive

-females have two X chromosomes, they get 2 copies of the allele

-so males are more likely to express recessive phenotype

inheritance of sex-linked characteristics

-males inherit these from the mother as father has Y chromosome so mother is carrier

autosome

any chromosome that is not a sex chromosome

autosomal linkage

when 2 or more genes are located on the same autosome

linked genes

-the closer together 2 genes are on the autosome the more closely they are linked - crossing over is less likely to split them so they stay together during independent segregation in meiosis and are more likely to be inherited together

why the observed ratio differs from phenotypic ratio

linked = if the alleles are linked, that means they mostly produced the more common alleles so there are lots of them but have still made the uncommon alleles due to crossing over (does not happen often so there is few)

dihybrid inheritance

where 2 phenotypic characteristics are controlled by 2 different genes

-if both heterozygous parents = 9:3:3:1

epistasis

where 2 non-linked genes interact to form phenotype with one gene masking the other gene

recessive epistasis

where 2 homozygous recessive alleles mask expression of another allele

-if homozygous recessive + dominant parents = 9:3:4

-example: yy = epistatic gene

dominant epistasis

where 1 dominant allele masks expression of multiple other alleles

-if homozygous recessive + dominant parents = 12:3:1

-example: W = epistatic gene

chi-squared test

used to compare observed and expected results in genetics to see if any difference between them is significant or not

criteria for chi-squared test

-data is in discrete categories

-large sample size

-no data values equal 0

-need to know degrees of freedom/classes and significance level

chi-squared formula

sum of (observed no. - expected no.)² / expected no.

chi-squared value

-if less than critical value, accept null hypothesis and any difference is due to chance + is not significant

-if greater than critical value, reject null hypothesis and accept experimental/alternative hypothesis so difference is significant