Inheritance Flashcards
define genotype
genetic makeup / allelic composition of an organism
define phenotype
physical manifestation of genetic trait that results from a specific genotype and its interaction with the environment
define wild-type allele
the most common allele in nature
define true-breeding
an organism that gives rise to all offspring o the same phenotype (fully homozygous, dominant or recessive)
describe the general steps of a genetic diagram
define alleles (let ? represent ?)
parental phenotype parental genotype
parental gametes
random fertilisation (punnett’s square)
offspring genotypic ratio
offspring phenotypic ratio
what is Mendel’s First Law of Segregation?
during the formation of gametes, the paired alleles segregate randomly so that each gamete receives one or the other with equal likelihood
what are reciprocal crosses, and what do they show?
crosses where parents “swap” genes yield same pattern of inheritance (eg. round male x square female VS square male x round female)
to see if genes are sex-dependent (if both crosses yield the same thing then they aren’t)
what occurs during a test cross, and what do the two results mean?
organism with dominant phenotype but unknown genotype crossed with homozygous recessive
all dominant offspring: homozygous dominant
1:1 dominant and recessive offspring: heterozygous
what are the four possible reasons for non-mendelian inheritance, and what offspring phenotypic ratio does each yield?
incomplete dominance (1:2:1)
codominance (1:2:1)
multiple alleles
lethal genes (2:1)
define incomplete dominance
neither of the two alleles is completely dominant to the other, heterozygote has an intermediate phenotype
define codominance
both alleles are equally expressed in heterozygotes (not an intermediate, that’s incomplete)
what is the similarity in phenotypic ratio and difference between incomplete dominance and codominance?
similarity is both are 1:2:1
in heterozygotes–
incomplete dominance: intermediate, new phenotype produced
codominance: not intermediate, both dom and rec phenotypes expressed tgth
what are lethal genes, and how does it affect the offspring phenotypic ratio?
arise from mutations
if lethal gene is…
recessive– 2:1 (homo dom : hetero; all homozygous recessive die)
dominant– only homozygous recessive survive lol
what is Mendel’s Second Law of Independent Assortment?
The segregation of one pair of alleles is independent of the segregation of other pairs
(during metaphase I)
what types of genes does Mendel’s Second Law not apply to?
linked genes
they do not assort independently bc they are linked on the same chromosome
what were the advantages of Mendel using garden peas to learn about inheritance?
distinct characteristics (discontinuous variation)
easy to cultivate
short life cycle of one year
large number of seeds produced
pollination easily controlled
pure breeds available
what is the phenotypic ratio of a dihybrid cross between two double heterozygotes?
9:3:3:1
what is the phenotypic ratio of a dihybrid cross test cross, between a double heterozygote and double homozygous recessive?
1:1:1:1
what are homogametic and heterogametic, and how do males and females call into this category?
homogametic females: only one type of gamete (egg with X chromosome)
heterogametic males: two types of gametes (sperm with X or Y chromosome, determines zygote’s gender)
what is sex / X- linkage in inheritance?
carrying of genes on sex chromosomes (more likely on larger X chromosome)
what are three examples of human X-linked disorders?
haemophilia, colour blindness, Duchenne muscular dystrophy
why do X-linked inheritance mainly affect males?
males only have 1 X while females have 2: neither homozygous nor heterozygous
hemizygous: only one copy of allele, will be directly expressed
females need two copies for recessive X-linked
what are the effects of Duchenne Muscular Dystrophy?
codes for enzyme that induces replacement of muscle by fibre, progressive weakness that affects muscle development
what is the purpose of a reciprocal cross, and what do its results show?
to tell if a trait is carried on a sex or autosomal chromosome, by reversing the traits of two parents
phenotype regardless of gender: autosomal
phenotype dependent on gender: sex-linked
list the four main mode of inheritance
autosomal dominant
autosomal recessive
sex-linked dominant
sex-linked recessive
what are the characteristics of a recessive inheritance?
if both parents are affected, all children should be affected
unaffected parents can produce affected individuals
trait often skips generations
x-linked: unaffected fathers only produce unaffected or carrier daughters
what are the characteristics of a dominant inheritance?
unaffected parents will not have affected children
trait should not skip generations
what is the difference between an autosomal and x-linked inheritance?
autosomal: males and females affected with equal probaility
x-linked inheritance:
most affected individuals are male
affected mothers = affected sons (always)
define linked genes
genes that control different characters and are situated on the same chromosome at different loci
if two genes are linked, what is not observed?
Mendel’s Second Law of Independent Segregation (9:3:3:1 and 1:1:1:1)
what is the difference between complete and incomplete linkage?
complete: no crossing over occurs between genes, only parental gametes
incomplete: some crossing over occurs, new combination of alleles in gametes
what test cross can be performed to detect gene linkage?
double heterozygous individual x double homozygous recessive
AaBb x aabb (expected; 1:1:1:1)
what are the three results of a gene linkage test cross, and what do they show?
diff chromosomes– 4 phenotypes 1:1:1:1
complete linkage: 2 parental phenotypes 1:1, no recombinants
incomplete linkage: 4 phenotypes, 2 parental 2 recombinant, more parental than recombinant (no fixed ratio)
what determines the linkage of linked genes?
relative distance between genes, probability of crossing over occurring between 2 gene loci increases with increasing distance between the 2 gene loci
in heterozygotes, what is a coupling or repulsion arrangement for linked genes?
coupling: AB/ab
repulsion: Ab/aB
what is the formula for crossover value (COV) or recombination frequency (RF)?
number of individuals with recombination OVER total number of offspring (in percentage)
what is cM, and what does it represent?
centiMorgan, distance between genes is equal to COV/RF (eg. 10% COV = genes are 10 cM apart)
what are the two components of phenotypic variance (Vp)?
Vp=Vg+Ve
Vg: genotypic variance
Ve: environmental variance
what are gene interactions?
two or more genes influence one particular character
commonly: metabolic pathway that controls development of one particular phenotype
define non-epistatic gene interactions
two unlinked, independently-assorting genes may interact to influence a single character
what is the difference between dihybrid inheritance and gene interaction?
dihybrid inheritance studies two characteristics controlled by two genes
gene interaction studies one characteristic controlled by two or more genes
define epistatic gene expressions
when the expression of an allele of one gene suppresses / inhibits the expression of alleles of a different gene
what are epistatic and hypostatic genes?
epistatic genes suppress / inhibit the effect of a different gene, recessively or dominantly
hypostatic genes are suppressed by epistatic genes
define recessive epistasis
two recessive alleles at the epistatic gene locus will suppress / inhibit the effect of either allele of the hypostatic gene at a different locus
how is recessive epistasis shown? (phenotypic ratio, what type of cross)
double heterozygous cross
9:3:4 phenotypic ratio
define dominant epistasis
one dominant allele at the epistatic gene locus will suppress / inhibit the effects of both alleles of the hypostatic gene at a different locus
how is dominant epistasis shown? (phenotypic ratio, type of cross)
12:3:1
double heterozygote
define duplicate recessive epistasis
two recessive alleles at either of the two gene loci will suppress / inhibit the effect of the dominant allele at the other locus
two epistatic genes, one hypostatic gene
how is duplicate recessive epistasis shown? (phenotypic ratio, type of cross)
9:7
double heterozygote
what are two types of variation?
continuous and discontinuous
compare continuous and discontinuous variation
discontinuous has discrete phenotypic classes without intermediates, continuous has range of phenotypes with intermediates
discontinuous controlled by single / a few genes, continuous controlled by multiple additive genes (polygenic inheritance), combined effect
discontinuous unaffected by envt, continuous affected by envt
what are the causes of genetic variation in a population (sexual and asexual)?
asexual: environmental
sexual:
crossing over between non-sister chromatids of homologous chromosomes during prophase I of meiosis
indep assortment of bivalents at metaphase plate during metaphase I of meiosis
random fertilisation
new alleles from mutations
give an example of how the environment affects the phenotype expressed
temp on himalayan rabbit colour
- homozygous for the ch allele of the tyrosinase gene
- which codes for a heat-sensitive form of the enzyme tyrosinase: needed for melanin production for black fur;
- heat-sensitive tyrosinase active only below 33°C
- black fur only on cool body parts eg extremities like ears
