Inheritance

Principles of heredity

established by Gregor Mendel: inheritance isn’t usually a blending of traits, each trait can be passed onto following generations

genotype

genetic makeup of an organism, alleles for a particular gene locus

phenotype

physical characteristics of an organism, expression of genes

dominant allele

Allele for trait expressed in phenotype, masks expression of recessive allele

recessive allele

allele for trait not expressed in phenotype, masked by dominant allele

trait masking

occurs at phenotype level, both proteins are created but one prevents the other from being visible

allele writing

capital letters for dominant alleles, lowercase letters for recessive alleles e.g Tt

incomplete dominance

one trait is not fully dominant over other trait. Phenotype is a mix of the two alleles

co-dominance

both alleles in genotype are fully expressed in phenotype

environmental factors

influence expression of alleles in genotype e.g availability of food changing height/weight of animal

Epigenetics

Alteration of allele phenotype by chemicals. doesn’t alter DNA code but can change external characteristics/what genes are expressed. changes can be inherited

Epigenome

Genes of an organism, plus attached chemical compounds that modify gene expression

single gene inheritance

each gamete only receives one of each allele, meaning diploid cells contain one allele from each parent

monohybrid cross

involves one pair of alleles for particular gene. Shows parents genotypes, and can predict offspring’s genotypes using a Punnet Square

mendellian ratio

two heterozygous parents will have a 3:1 dominant-recessive phenotype ratio in their offspring

test cross

cross between individual w/ unknown genome and homozygous recessive individual

test cross results

homozygous dominant parent=all dominant phenotypes in offspring, heterozygous parent=half dominant, half recessive phenotypes in offspring.

Lethal genotypes

cause phenotypes resulting in death. Can be homozygous recessive or homozygous dominant. alter predicted ratio of offspring

multiple alleles for one gene

still only 2 alleles for pair of chromosomes, but have more types of alleles and combination possibilities

ABO blood types

multi-allelic trait, A and B co-dominant, O is recessive

X-linked genes

700 genes determining non-sex characteristics, may have alternative alleles and show dominant/recessive pattern

x-linked inheritance

X and Y are not homologous: genes on X not carried on y. This means males only have one allele for x-linked trait, and only females can be carriers

Y-linked genes

Most genes are linked to aspects of maleness e.g sperm production

colour blindness

example of an X-linked trait, affects 1 in 12 males vs 1 in 250 females. (x-linked traits more common in males)

Pedigrees

squares=male, circles=female, horizontal line=marriage, vertical line=offspring, coloured shape=affected, roman numerals=generations

autosomal recessive

can skip a generation, two unaffected parents may have affected child, child must receive recessive allele from each parent

autosomal dominant

children must have at least one parent with trait, two affected parents may have unaffected child

x-linked dominant

affected father will pass trait onto all daughters but no sons, does not skip generations

x-linked recessive

affected mother passes trait to all sons but no daughters, more affected males, may skip generation.

Dihybrid cross

study of two different genes at same time, if on different chromosome pairs=show independent assortment

dihybrid cross notation

paired letters next to each other e.g XxYy, gametes written at top of square, gametes can only be different genes

dihybrid test cross

determines genotype of unknown individual with dominant traits. crossed with homozygous individual for both genes

heterozygous parent test cross

9:3:3:3:1, F1 phenotype ratios

homozygous parent test cross

1:1:1:1, F1 phenotype ratios

linked dihybrid crosses

dihybrid crosses for two genes on same chromosome. genes do not assort independently and so results are different

linked gene notation

gametes are written with seperate alleles together e.g AB, and join in offspring like this: ABAB

crossing over

sections of chromosomes exchanged in meiosis, means linked genes do not always stay together.

recombinant

chromatids that have exchanged genes in crossing over process

polygenic inheritance

characteristics controlled by alleles of more than one gene

continuous variation

produce a smooth transition of indistinct traits, usually quantitive e.g height

discontinuous variation

has only a few distinct phenotypes, usually qualitative