D3.2 Inheritance HL

segregation

the pair of alleles of each parent separate and only one allele passes from each parent on to an offspring, determined by random orientation of chromosomes in metaphase I

• occurs independently in unlinked genes found on different chromosomes

dihybrid cross

investigates inheritance of two traits controlled by two genes, whether inherited together or independently

• application of law of segregation and independent assortment

Mendel’s law of independent assortment

assortment of one pair of genes into gametes is independent of the assortment of another pair of unlinked genes, determined through performing dihybrid crosses

9:3:3:1 ratio

received when parents in a dihybrid cross that are heterozygous for two genes are crossed together

reasons for irregular ratios after dihybrid cross

• genomic imprinting

• codominance

• sex-linked genes

• linked genes (carried close together on the same chromosome)

• non-heterozygous parents

• gene interactions (one gene affects expression of another)

locus

a gene’s specific position on one of 22 types of autosome or one of two types of sex chromosome

linkage group

all genes with loci on the same chromosome

crossing over

• two homologous chromosomes, one bearing alleles A and B and the other alleles a and b are paired at prophase I

• two nonsister chromatids undergo crossing over, causing portions of each to exchange places

• results in two recombinant chromatids, with alleles A and b on one chromatid and a and B on the other, leads to added genetic variation

closer proximity of linked genes

lower likelihood of transferring alleles from one chromosome to another and creating recombinant gametes

recombinant

chromosome or DNA with a new combination of alleles due to crossing over in meiosis, different from that of original parents

IB gene linkage notation

• line represents chromosomes

• locus 1

• locus 2

backcross/test cross

tests frequency of recombination between two genes by crossing heterozygous individual with homozygous recessive

backcross of unlinked genes

equal ratio of four potential phenotypes

backcross of linked genes

two phenotypes in high amounts, two phenotypes in low amounts

chi-squared test

• calculate expected frequencies, assuming independent assortment, for each of the four phenotypes (expected frequency = expected probability * total)

• determine degrees of freedom

• calculate critical value using table and 0.05 confidence

• calculate chi squared value

• if chi squared value greater than critical value, null hypothesis rejected

chi-squared test null hypothesis

alleles assort independently and are not linked, statistically insignificant

• rejected if chi-squared value greater than critical value

chi-squared test alternative hypothesis

alleles do not assort themselves independently, genes are linked

• accepted if chi-squared greater than critical value

chi squared degrees of freedom

total number of classes - 1

chi-squared value

∑(observed-expected)²/expected