Genetics Exam 2

Lethal Alleles

may modify phenotypic ratios if they result in death before offspring can be evaluated/counted

Gene Interaction

more than one gene influences a characteristic

Epistasis

one gene modifies or prevents expression of another gene

Bombay Phenotype

H gene, when hh, masks the expression of ABO gene

Recessive Epistasis

a gene's expression is masked/suppressed by a second gene's alleles (9:3:4 phenotypic ratio)

Dominant Epistatsis

a gene's expression is masked by a second gene's dominant allele (12:3:1 phenotypic ratio)

Complimentary gene interaction

Both genes are needed for expression, which is then masked by recessive alleles for either gene (9:7)

Compound Heterozygote

An individual who is heterozygous, but for two different mutant alleles of the same gene, and both alleles give the same mutant phenotype

Pleiotropy

Expression of a single gene has multiple phenotype effects

X-linkage

occurs when traits are controlled by genes on the X chromosome

Sex-Limited Inheritance

Expression of specific phenotype is absolutely limited to one sex

Sex-influenced inheritance

The phenomenon in which an allele is dominant in one sex but recessive in the other

Penetrance

the percent of individuals with a mutant genotype that to at least some degree, express the associated mutant phenotype

Heterochromatin

chromatin that is highly packaged and condensed, if a gene is relocated to this region, its expression may be decreased

Extranuclear Inheritance (cytoplasmic inheritance)

involves genes not found in the nucleus but rather in the mitochondria/chloroplasts (cytoplasmic organelles). only from the female parent.

Heteromorphic

sex chromosomes characterize one sex or the other in a wide range of species (such as XY males among mammals).

Nondisjunction

failure of chromosomes to properly segregate in meiosis or mitosis

Mosaic

individuals with an abnormal number of chromosomes in some but not all cells due to nondisjunction in mitosis.

Klinefelter Syndrome

A chromosomal disorder in which males have an extra X chromosome, making them XXY instead of XY.

Turner Syndrome

A chromosomal disorder in females in which either an X chromosome is missing, making the person X instead of XX

Triple X Syndrome (47, XXX)

three X chromosomes along with a normal set of autosomes in females

47, XYY Condition

about 1 in 1000 male births

Pseudoautosomal Regions (PAR)

present on both ends of the Y chromosome and share homology with regions on the X chromosome

male-specific region of the Y (MSY)

Region of the Y chromosome that has little homology with the X chromosome and does not synapse

Sex-determining region Y (SRY)

critical gene on the Y chromosome that controls sexual development

Barr Bodies

Highly condensed and inactive X chromosomes. In 46, XX females, it arises from one of two X chromosomes

The Lyon Hypothesis

which X remains active and which is inactivated is random in each cell. Once an C is inactivated, the same X remains inactive in all daughter cells, resulting in a patch (calico cats)

Calico Cat

Amount if white is due to an autosomal gene and the gene for orange/black is X-linked. It has different X chromosomes inactivated in different cell lines, resulting in black and orange patches.

Imprinting

the process whereby expression of genes on one homolog, but not the other, is affected

Epigenetic

pattern of expression influenced by reversible chemical modifications to DNA/chromatin and not the sequence of bases/nucleotides.

Genic Balance Theory

In Drosophila, sex is determined by the ratio of X-chromosomes and autosomes.

temperature-dependent sex determination

Sex is determined/influenced by temperature during embryonic development

Chromosome mutations

Includes a change in the total chromosome number, deletions/duplications, or rearrangements of genetic material. Changes can result in phenotypic variation and may be lethal

Aneuploidy

organism gains/loses chromosomes and has other than an exact multiple of their haploid set

Euploidy

complete haploid sets of chromosomes are present (but not necessarily the typical number of haploid sets) Example: 46, XX

Polyploidy

more than two haploid sets of chromosomes are present

Nondisjunction

Paired homologs in meiosis I or sister chromatids in meiosis II fail to disjoin and move to opposite poles. Disrupts the normal distribution of chromosomes into gametes

Monosomy

loss of one chromosome (2n-1)

Trisomy

addition of one chromosome (2n + 1)

Downsyndrome

due to trisomy 21 (47, 21+). Most frequently occurs due to nondisjunction of chromosome 21 in meiosis.

Maternal Age Effect

Increased frequency of nondisjunction with an increasing age of the female parent

autosomal trisomies that survive to term

• Trisomy 21 (47, 21+) down syndrome

Patau Syndrome

Due to having an additional copy of chromosome 13 in some or all of the body’s cells (47, 13+)

• Trisomy 18 (47, 18+) Edward syndrome

Polyploidy Naming

based on the number of haploid sets of chromosomes

• Triploid has 3 sets (3n)

• Tetraploid has 4 sets (4n)

• Hexaploid has 6 sets (6n)

two ways that polyploidy originates

autoploidy and alloploidy

Autoploidy

Each additional set of chromosomes is identical to the parent species. (triploids AAA, tetraploids AAAA, etc.)

Autotriploids

(3n) arise due to failure of all chromosomes to segregate during meiosis. Two sperm fertilizing an ovum/egg. Due to increased fertility with an even number of chromosomes

Autotetraploids

Have four sets of chromosomes (4n) that are all from the same species; theoretically more likely to be found in nature than autotriploids.

Autopolyploid flowers

often have increased size

Allopolyploidy

Combination of chromosome sets from different species ---- consequence of interspecific matings (hyrbidization of two closely related species.) hyrbid offspring may be sterile

Cri du Chat syndrome

"cry of the cat" results from a small terminal deletion of chromosome 5's small arm. It is not inherited --- it results from sporadic loss of chromosomal material in gametes. (46, 5p-)

Duplication

May arise through unequal crossing over between synapsed chromosomes during meiosis which also results in deletion

Inversion

segment of chromosome turned 180 degrees within chromosome -- no loss of genetic information

Paracentric

inversion does not include the centromere

Pericentric

inversion does include the centromere

inversion heterozygotes

individuals with one inverted and one non-inverted chromosome. Often have a normal phenotype, but may have reduced fertility due to inviable gametes/progeny

Dicentric chromatid/chromosome

two centromeres

Acentric chromatid/chromosome

lacking a centromere

Translocation

movement of chromosomal segment to new location in the genome

Robertsonian translocation

fusion of non-homologous acrocentric chromosomes at their centromeres and loss of short arms. Familial down syndrome is usually due to this.

Fragile Sites

poorly stained chromosome areas and may be regions where chromatin is not tightly coiled. Susceptible to breakage when cells cultured without folic acid.

Fragile-X syndrome

most common form of inherited intellectual disability. Individuals have a fragile site on the X-chromosome

Linkage

exception to independent assortment

Linked genes

genes physically located on the same chromosome. Tend to segregate and be inherited together as a single unit

heteroplasmy

In four-o’clock plants, variegated leaves are are a sign of ___________

penetrance

Consider an autosomal recessive disorder where only 70% of homozygous recessive individuals show any symptoms of the disorder. This is an example of variable/incomplete:

variable expressivity

Fraternal twin sisters with normal karyotypes are homozygous for an autosomal recessive disorder. While one twin has a mild phenotype and is only slightly affected by the disorder, the other twin’s phenotype shows severe effects of the disorder. If epistasis is not involved, which of the following might account for the phenotypic differences?

Y chromsome

has the SRY gene, has heterochromatic regions on its q arm, and has some homology with the human X chromosome

6:3:3:4 ratio

F2 phenotypic ratio that suggests two genes

ZW genotype

genotype for a female bird

The Frequency of crossing over is …

… proportional to the distance between loci

Barr Bodies Per Cell

N-1 rule, where N is the number of X chromosomes (Turner Syndrome - 45, X would have no barr bodies and Klinefelter syndrome - 47, XXY would have one barr body)

nondisjunction in meiosis II

has the best chance of producing some euploid offspring upon fertilization with a normal gamete

bar-eye phenotype

Narrow, slit-like eyes in the fruit flies due to a duplication of the 16A region of the X chromosome (making the trait X-linked)

Familial Down Syndrome

due to a Robertsonian translocation. The familial down syndrome translocation carrier has 45 chromosomes, but is phenotypically normal. Individuals with familial down syndrome have 46 chromosomes.

12q23 gene is located here

Region 2 of chromosome 12’s long arm

Unequal Crossing Over

Most likely will occur in meiosis I

Colchicine

May be used to produce polyploids

maternal disjunction in meiosis I

Down syndrome is most often the result of:

Independent Assortment

Genes Assort independently if on different chromosomes, large numbers of meiotic events produce four genetically different gametes in equal proportions (1:1:1:1 ratio).

Complete Linkage

No crossing over is observed between loci, only 2 genetically different gametes are produced in equal proportions

Incomplete Linkage

crossing over is observed between the loci. Involves two non sister chromatids and generates 4 genetically different gametes

Chiasmata

Points of genetic exchange.

As two genes are located closer together, they are less likely to have chiasmata form between them and to have crossing over between them.

Recombination Frequency

related to genetic distance between genes on a chromosome - 1 map unit = 1 centimorgan.

RF (%) = number of recombinants/total number of progeny

Coupling Phase

Linked heterozygous gene pairs in the arrangement AB/ab. Parental type has both dominant alleles on one homolog, and both recessive alleles on the other homolog

Repulsion Phase

Linked heterozygous gene pairs in the arrangement Ab/aB. parental type has one mutant allele and one wild-type allele on each homolog.

Single Crossovers

genes far apart are more likely to have a crossover between them than genes close together. In incomplete linkage, the number of crossover/recombinant gametes/progeny approaches but does not exceed 50%.

Rule of Thumb when classifying progeny as parental vs recombinant

the majority are parental and minority are recombinant because RF does not exceed 50%

Single Crossovers

used to determine the genetic distance between two linked genes

Gene Mapping

as distance between two genes increases, mapping estimates become more inaccurate, Need at least 3 genes to detect a double crossover.

coupling phase

Genes A and B are on the same chromosome. You make an AABB x aabb cross to produce F1 progeny. With respect to genes A and B, the F1 progeny are likely to be in:

Determine recombination frequency between genes that are close together, but not completely linked

For genes on the same chromosome, which of the following will likely give the most accurate estimate of genetic distance?

on the same chromosome

Genes are most likely to be linked if they are on:

those that are far apart on a chromosome

Which of the following pairs of genes are most likely to have a crossover between them?

crossing over

Can make new combinations of alleles on a chromosome

Recombination in a parent heterozygous for all genes

most likely to generate chromosomes with new combinations of alleles