Genetics 311 Indiana University

Pseudoautosomal regions (PARs)

Homologous segments on the tips of the X and Y chromosomes that allow them to pair and recombine during meiosis. The two PARs contain about 30 genes total.

SRY

A protein that activates testes development and prevents the formation of ovaries. Sex determining region of Y.

Sex Reversal

(XX with SRY+ is male) and (XY with SRY- is female)

Sex Chromosomes

X(large amount of protien-coding genes) and Y(much smaller amount)

Thomas Hunt Morgan

Established a firm experimental base for the chromosome theory. (Specifc genes reside on specific chromosomes.

Crisscross inheritance

Males inherit eyecolor mutation from their mothers.

X-Linked

The gene is carried by the X chromosome and the Y chromosome carries no allele of the gene. 

w+

normal / wild type allele

w

mutant allele

hemizygous

an individual who has only one member of a chromosome pair or chromosome segment rather than the usual two. Often used to describe X-linked genes in males who have only one X chromosome.

Nondisjunction

where the X chromsomes fail to seperate during meiosis in fmeales. Failure in chromsome segregation. Occurs in meiosis I or meiosis II.

All possible products of nondisjunction + fertilization

XXY

XXX

XO

OY (these die in embryonic development)

Kinetochore

A structure in the centromere region of each chromatid that is specialized for conveyance. Each kinetochore contains protein that enable the chromsome to slide along the microtubule.

Pedigree

A diagram of a family’s relevant genetic features, extending back through as many generations as possible.

Family pedigree diagram symbols

Squares = males

Circles = females

Diamonds = unspecified sex

Late-onset genetic trait

Symptoms are not present at birth and manifest themselves later in life. example = huntington disease

Incomplete Dominance

Codominance

Variable Expression

Incomplete penetrance

monohybrid

experiments involving hybrids for only a single trait. Cross of pure breeding parents that will always produce f1 hybrids that resemble one of their parents.

reciprocal cross

reversing the characteristics of the male and female parents, thus controlling whether a particular characteristic was transmitted via the eggs or sperm.

Dominant

characteristic that appears in all f1 hybrids

recessive

characteristic that remains hidden but reappears in F2

Punnet square

visual summary of a cross

principle of segregation

two identical alleles of pure breeding organism segregate during gamete formation. Each sperm or egg carries only one of each pair of parental alleles.

Hybrid

combining two different organisms with seperate characteristics. Offspring of parents that are genetically different.

F1

first generation of offspring

F2

second generation of offspring

phenotype

physical, visble, observable quality

genotype

pair of alleles present in an indivdual

gene

the heritable entity that determines a trait

alleles

alternate forms of a gene

homozygous

has two identical alleles of a given gene

heterozygous

two different alleles of the same gene

test cross

the cross of an individual of ambigous genotype with a homozygous recessive individual.

dihybrid cross

the seperation of the two alleles of a gene into different gametes

addition rule

mutually exclusive events (example is “or” events)

multiplication rule

independent events (example is “and” events)

somatic cells

non-reproductive body cells

germ cells

reproductive cells located in testes and ovaries.

Diploid

zygote carrying two matching sets of chromosomes

haploid

gametes of a diploid organism

mitosis

the nuclear division that apportion chromosomes equally to two daughter cells.

meiosis

chromosomes replicate once but the nucleus divides twice.

interphase

ongoing protein synthesis and cell growth

cytokinesis

this splits the nucleus and finally makes the identical daughter cells in mitosis

nuclear envelope

seperates nucleus from cytoplasm and protects the genetic material

nucleolus

responsible for synthesis of ribosomes

prophase (mitosis)

1.Chromsomes condense and become visible

2. centromeres move apart toward opposite poles and generate new microtubules

3. nucleoli begin to disappear

chromatid

the rods that make up chromosomes

centromere

holds chromosomes together.

1 chromosome = 1 centromere

0 centromere = no chromosome

metaphase (mitosis)

1. chromosomes align on the metaphase plate with sister chromatids facing opposite poles 

mitotic spindle

microtubule based structure that segregates replicated chromosomes.

metaphase plate

the line where chromosomes align to eventually seperate in anaphase

anaphase (mitosis)

1. the connection between the centromers of the sister chromatids is severed.

2. The now seperated sister chromatids move to opposite poles

Prophase I (meiosis)

made of five stages, where homologues condense, pair, and crossing over occurs.

Stages:

1. Leptotene

2. Zygotene

3. Pachytene

4. Diplotene

5. Diakinesis

Metaphse I (meiosis)

1. Tetrads line up along the metaphase plate

2. Each chromosome of a homologous pair attaches to fibers from opposite pole

3. Sister chromatids attach to fibers from the same pole.

Leptotene

1. Chromosomes begin to condense

2. Centrosomes begin to move toward opposite poles

Zygotene

1. Homologus chromosomes enter synapsis

2. The synaptonemal complex forms

Pachytene

1. Synapis is complete

2. Crossing over, genetic exchange between nonsister chromatids of a homologous pair occurs.

Diplotene

1. A synaptonemal complex dissolves

2. A tetrad of four chromatids is visible

3. crossover points appear as chiasmata, holding nonsister chromatids together

4. meiotic arrest also occurs at this time in many species

Diakinesis

1. Chromatids thicken and shorten

2. The nuclear membrane breaks down and the spindle begins to form

Zygote

the cell formed by the fertilization of the egg by the sperm during sexual reproduction; in humans, eggs and sperm are haploid, and zygotes are diploid.

Anaphase I (meiosis)

1. sister centromeres remain connected to each other

2. the chiasmata dissolve

3. homologous chromosomes move to opposite poles

Synapsis (zipper = synaptonemal complex)

matching chromosomes are zipped together

Telophase I (meiosis)

1. The nuclear envelope reforms

2. Resultant cells have half the number of chromosomes, each consisting of two sister chromatids.

3. Cytokinesis seperates the daughter cells

Tetrad / bivalent

synapsed chrosome that contains four chromatids

synapsed chromosome that contains two chromatids

chiasmata

A point in which paired chromosomes remain in contact during the first metaphase of meiosis. Crossing over + exchange of genetic material occurs.

Sister chromatids

two identical DNA molecules that stay attached to centromere until cell replication.

Pedigree

an orderly diagram of a family’s relevant genetic features, extending through as many generations as possible

Pleiotropy

phenomenon in which a single gene determines a number of distinct and seemingly unrelated characteristics

Central dogma

flow of genetic information

Complementation test

if progeny are all wildtype = the strains had mutations in different genes (complementary)

if progeny are all mutant = the the strains had mutation in the same gene (not complementary)

Happloinsufficient 

Abnormal phenotype shown because the level of gene activity is not enough to produce a normal phenotype

Polar body

small haploid cell asymmterical to the larger secondary oocytese

Epigenetic phenomena

heritable, self perpetuating changes in gene expression not caused by mutation in the base-pair sequence of DNA.

silenced

a promoter is repressed long term

cellular memory

transmission involves collaboration between

1. modified cytosine residues in DNA

2. Modified histone tail amino acids in chromatin

3. Small RNA molecules

Epigenetic information

the combination of molecular factors that determines whether a gene is “on” or “off”

DNA methylation

a biochemical modification of DNA in which a methyl group is added to the fifth carbon of the cystosine base in a 5’ CpG 3’ dinucleotide pair on one strand of the double helix.

DNA methyltransferases (DNMTs)

enzymes that catalyze the methylation of cytosines in CpG dinucleotides

CpG islands

DNA sequences that are hundreds to thousands of bp long where the frequency of CpG nucleotides is much higher than the rest of the genome. When unmethylated, the chromatin is open and the gene is transcriptionally active.

meCPs

repressors that bind to methylated CpG islands and close the chromatin

epistasis

a gene interaction in which the effects of alleles at one gene hide the effects of alleles at another gene

suppression

to inhibit a genes expression or activity, often by another gene or mechanism.

sex-limited

traits that are expressed only in one sex, even though both sexes carry the alleles responsible for the trait.

sex-influenced

traits that are expressed in both sexes but are influenced by the sex of the individual, often exhibiting different patterns of expression.

genetic anticipation

occurence of genetic disorder at progressively earlier ages in subsequent generations

genomic imprinting

the phenomenon in which a gene’s expression depends on the parent that transmits it; caused by sex-specific DNA methylation.

conditional mutation

A mutation that has the wild-type phenotype under certain (permissive) environmental conditions and a mutant phenotype under other (restrictive) conditions. Examples are temperature sensitive and nutritonal effect

acquired characteristics

the belief that traits influenced by environment can be passed on to offspring

chromosome or genetic map

a diagram showing the location of genes on a chromosome

recombination

when crossover occurrs between genetic markers

crossover

exchange of corresponding chromosome segments between homologs

cis

the action of a dna site or an RNA molecule that acts only on the DNA or RNA to which it is connected physically

trans

the action of a protein or RNA that can bind to target sites on ANY DNA or RNA in the cell, connected or unconnected

parental

same combination of alleles found in parent’s chromosomes

nonparental

different combination of alleles from parents, i.e. recombinants

locus

a designated location on a chromosome; sometimes refers to a gene