Genetics Exam 2

Complete Dominance

Phenotype of the heterozygote is the same as the phenotype of one of the

homozygotes.

Why is a three point test cross better than a two point

you only need one cross to find the gene order and you can identify any double crossovers

heteroplasmy

two or more genetic variants in a cell due to cytoplasmic inheritance

expected double crossovers equation

recombinatin frequency 1 x recombination frequency 2 x number of progeny

Incomplete dominance

Phenotype of the heterozygote is intermediate (falls within the range) between the phenotypes of the two homozygotes. (a pink flower)

codominance (name the 3 classic examples)

Phenotype of the heterozygote includes the phenotypes of both homozygotes. (a mixed red and white flower, calico cat, AB blood type)

penetrance, 3 reasons for incomplete penetrance

percentage of individuals that express the expected phenotype for a genotype (example your geneone encodes for a mutation that causes you to have an extra toe, but you actual don’t because that gene doesn’t have full penetrance)

90% penetrance: 9/10 people have the expected phenotype

caused by effects of other genes, environmental, epigenetics

expressivity, 3 causes for limited expressivity

degree to which a trait is expressed (example, incomplete expressivity is when you just have an extra nub instead of a full extra finger)

caused by effects of other genes, environmental, epigenetics

formula for possible genotypes at a locus with n alleles

1 + 2 + 3 + 4 +…..n

epistasis

one gene masks the effect of another gene

epistatic: gene that does the masking

hypostatic: gene that gets masked

gene interaction

effects of genes at one locus depend on the presence of genes at other loci

complementation

genetic test used to determine whether two mutations are on the same gene. Done by crossing two homozygous individuals for dif mutation. Offpsring will be heterozygous.

If offspring exhibit mutant phenotype, the mutation occurs at the same locus (since at that locus they can only have two alleles so it will be one allele of each mutation)

If offspring exhibit wild type, the mutations occur at different loci/genes (since they can inherit the normal alleles AND the mutant alleles from each parent)

sex influenced characteristics

traits whose expression differs between male and female, but is still presented in both. (ex. heterozygous male will show the trait vs only homozygous recessive female will show the trait)

autosomal!!!

sex limited characteristics

traits where gene are present in both sexes but only expressed in one. (ex. present in males but female can still pass it down)

autosomal

sex linked characteristics

traits encoded by genes on the sex chromosomes

ctyoplasmic inheritance

transmission of traits controlled by genes located outside the nucleus, typically in mitochondria found in the cytoplasm. Inherited from only one parent. When mitochondria replicate and go into different cells in cell division, some have a wild type gene and some have a mutant. Therefore, one daughter cell could get a wild type gene and the other could get a mutant.

can be chloroplastic inheritance but is usually mitochondrial inheritance

genetic maternal effect

genotype of the mother determines phenotype of the offspring

epigenetics and genomic imprinting

• Epigenetics: phenomena due to alterations

to DNA that do not include changes in the

base sequence; often affect the way in which

the DNA sequences are expressed

• Genomic imprinting is a type of epigentics:

the process of genes having different expression depending on if the transmitting parent is male or female

anticipation

a genetic trait becomes more strongly expressed at an earlier stage as it is passed from generation to generation (expressed later in life in earlier generations, expressed at beginning of life in later generations)

occurs due to expansion of an unstable region of DNA from generation to generation

phenocopy

environmental factors alone causing a phenotype that is the same as a phenotype made by genetic factors

pedigree

a figure representing family history and inheritance of one of more characteristics

aspects of an autosomal recessive trait

appears in both sexes equally, tends to skip generations, affected offspring usually have unaffected parents, appears in incest

autosomal dominant trait

will appear in about half the offspring if a heterozygote and recessive homozygote are crossed, appear equally between sexes, affected people have at least one affected parent, unaffected people don’t transmit

will only skip generations if there is incomplete penetrance or a mutation

x linked recessive trait

affected male does not pass to sons, females need it on both x chromosomes, male can pass allele to daughter, affector or carrier mother will always give it to son

x linked dominant trait

will not skip generations, affected males pass it to daughters but not sons, males only get it from mother. Heterozygous mothers only pass it so sons half the time

y linked trait

only males can get it, all sons of affected fathers have ir

what will offspring be like with inheritance of two linked genes with no crossing over

shows no new combination of traits. Offspring have same genotypes as 1 of their parents. Is nonrecombinant. Alleles dont separate in meiosis.

inheritance of two genes via indep. assortment

found in genes not on the same chromosome, alleles separate and recombine independently, so half are recombinant and half are not.

inheritance of two genes via linkage with crossing over

incomplete linkage. genes are on the same chromosome but far enough that crossing over sometimes occurs. Recombination frequency is somewhere between 0 and 50%. Usually it is closer to 0 (there are more nonrecombinant progeny)

recombination frequency (and what other calculation is it half of)

percentage of offspring that show recombinant combinations of traits compared to the parental combinations, measures how often crossing-over happens between two genes during meiosis.

Is half of the crossing over frequency

(genes farther apart are more likely to cross over)

coupling

cis configuration. alleles are arranged on homologous chromosomes in a way that dominant alleles are on one and recessive are on the other.

repulsion

trans configuration. alleles are arranged on homologous chromosomes in an a way that dominant and mutant alleles are found on the same chromosome.

genetic mapping, what helps us do it

we can study recombination frequency to make a gene map, since distance between genes is proportional to how frequently they recombine

(50% recombination genes wont be on the map bc they arent on the same chromosome)

double crossover

happens with two crossover sites in homologous chromosomes. The only recombined parts of the chromosome will be between the two crossover sites, everything outside of that is unchanged because the second crossover reversed the first

(this is why gene B in a three point test cross is the only gene altered in a double crossover)

three point testcross, and the three types of crossovers

used to gene map/find the order of three genes. It is better than two points because double crossovers can be recognized.

• crossover between 1st and 2nd gene

• crossover between 2nd and 3rd gene

• double crossover between both 1 and 2 and 2 and 3

how to determine gene order from a 3 point cross

1. Identify the parental phenotypes (remember a heterozygous and homozygous recessive parent are crossed in a testcross). They will be the most numerous, since crossing over does not happen very often between genes close together. NOT ALWAYS THE SAME GENOTYPE AS THE PARENTS ITS THE MOST NUMEROUS

2. Identify the two least numerous phenotypes. These are the double crossover progeny. They are least numerous because crossing over if not super common, even less common to happen twice.

3. Compare the nonrecombinant and double crossover progeny. They should be alike in two alleles and differ in one.

4. The allele that differs is the one encoded by the middle gene.

coefficient of coincidence, why is the result usually not 1? (crossovers are not….)

observed double crossovers / expected

shows the percent of the expected DCO that is actually observed

can happen because crossovers are not independent events and can interfere with eachother

interference

degree to which 1 crossover interferes with additional crossovers in the same region

1-cc , get the percent of expected DCO progeny that will not be observed