Intro to Genetics- Exam 3 (All Materials)

When were sex chromosomes first identified?

The 1900’s

Chromosome Theory of Inheritance

Genes are on our chromosomes

Protenor Mode

• XX/X0 sex determination

• Depends on random distribution of X chromosome into half of male gametes

• Sex determined by number of X chromosomes

  • Female = 2

  • Male = 1

• example: butterflies

Y Chromosome Mechanism

• Presence of the Y chromosome determines sex of organism

• Males are heterogametic (XY)

• Females are homogametic (XX)

• example: humans and other mammals

Primary Pseudoautosomal Region

• PARs Region

• Found in sex chromosomes of XY or ZW species

• At the tip of the short arms of chromosomes

Secondary Pseudoautosomal Region

• Other PARs region

• At the tip of the long arms of the sex chromosomes

PARs

• <5% of genetic material

• Where the X and Y chromosomes are similar

MSY

• Male Specific region of the Y chromosome

• Contains a number of genes specific for male development, including SRY

• 2 subregions:

  • Active genes (euchromatin)

  • No active genes (heterochromatin)

image on pg.6 of sex determination powerpoint

SRY

• Sex-determining region Y

• Encodes “testis determining factor”

ZW System

• Male is the homogametic sex

• Males are ZZ

• Females are ZW

• Example: chickens

X Chromosome-Autosome Balance System

• The ration of X chromosomes to the number of haploid sets of autosomes determines sex

• Studied by Calvin Bridges

• Presence/absence of the Y does not play a role

• Example: flies, worms

X Chromosome-Autosome Balance System Ratios

• 2:2 = female

• 1:2 = male

• 3:2 = sterile female

• 3:4 = sterile intersex

• 2:3 = sterile intersex

• 1:2 = sterile male

Dosage Compensation

• Either multiples of a chromosome (e.g. X chromosome-autosome balance system) or one gender has more of a chromosome than the other (e.g. XX/XY and ZZ/ZY sex determination)

• Inactivating multiples of a chromosome to ensure gametes get an equal amount of sex chromosomes from each parent.

• ex. XX worms transcribe X-linked genes at half the rate of XO males

TSD: Temperature-dependent se determination

• Sex is determined by the incubation temperature of the eggs at a critical period in embryotic development

• ex. some reptiles, some lizards, most turtles

Arrangement of sex organs in plants

• Dioecious species (e.g. ginkgo) male and female parts on separate plants

  • Not capable of self-fertilizing

  • Some have sex chromosomes and use the X chromosome-autosome balance system (many other sex determinations in these plants also occur)

• Monoecious species have male and female parts on the same plant

  • Capable of self-fertilizing

Nondisjunction

• Failure of homologs to separate in meiosis I

• Failure of sister chromatids to separate in meiosis II

• Leads to aneuploid cells and/or gametes

  • Failure of separation can happen in mitosis also

Aneuploidy

Chromosome count deviates from the normal chromosome compliment (too few or too many)

examples: turner syndrome (missing second sex chromosome), klinefelter syndrome (extra X)

Barr Bodies

• Observed in human aneuploid embryos to inactivate extra X chromosomes

• Number = number of total X chromosomes - 1

Lyon Hypothesis

• Inactivation of X chromosome occurs early in development

• Once inactivation occurs, all progeny cells have the same X chromosome inactivated

• Leads to patches of tissues with different phenotypes

• example: calico cats

Pedigrees

Typically constructed to trace the inheritance of a specific medical condition

Pedigree- circle

Female

Pedigree- square

male

pedigree- shaded

affected (expresses the trait)

pedigree- unshaded

unaffected (does not express the trait — CAN BE A CARRIER)

Pedigree- sibling order

Siblings are placed in birth order (if known) from left to right

Pedigree-parental passing of dominant traits

• If the male parent expresses the traits, the dominant allele is passed to all XX female offspring

• If the female parent expresses the trait, the dominant allele is passed to all offspring

• The trait will be seen in all generations

Pedigree-parental passing of recessive traits

• If the female parent expresses the recessive trait, a recessive allele will be passed to all offspring (XY males will express, XX females will carry)

• If the male parent expresses the recessive trait, all XX female offspring will be carriers, XY males will not express or carry the recessive allele.

Alleles

Alternative forms of a gene

Mutation

Change in DNA sequence results in new phenotypes (and silent mutations that don’t get expressed)

Wild-type (wt) allele

Occurs most frequently in nature and is usually, but not always, dominant. Indicated by italic letter and superscript

Dominant alleles

Indicated by either an upper case letter or multiple letters

No dominance exists

Italic uppercase letters and superscripts to denote alternative alleles

Recessive alleles

Indicated by either an italic lowercase letter or an italic letter/group of letters

Mutant alleles

Written in a variety of ways: italic letter, minus sign, assigned number (ex. CFdel508)

Multiple alleles

More than 2 alleles exist in a population. Denoted with the same symbol and distinguished by different superscripts. More than 2 phenotypes are expected (ex. ABO blood groups)

Antigens

Surface of RBCs

• Interact with specific antibodies in blood sera, this could result in agglutination (clumping)

• RBCs have a variety of surface markers. Blood type is just one of these markers.

H substance

• One or two terminal sugars are added

• O blood types (ii) only have the H substance protruding from RBCs

A and B antigens

Carbohydrate groups added to the H substance

Antibodies

Proteins in blood sera. The present varies with ABO blood phenotype. They interact with specific antigens on RBCs.

• A: anti-B

• B: anti-A

• AB: no antibodies

• O: anti-A and B

(If you are type A, anti-A antibodies will clot your blood)

RBC Maturation Process

They start as a stem cell and develop into a mature RBC

FUT Gene

Adds fucose (green) to the end of the H precursor substance

Complete Dominance

The heterozygote phenotype is that of the homozygous dominant

Codominance

The heterozygote expresses both homozygote phenotypes at the same time

• No dominant allele (blue allele + white allele = blue and white phenotype)

Incomplete (partial) dominance

Refers to the phenotype of a heterozygote that is intermediate between the phenotypes of the two homozygotes

• No dominant allele (red allele + white allele = pink phenotype)

Do variations on dominance relations refute Mendel’s Principle of Segregation?

No.

Dominance relations affect phenotype and have no bearing on the segregation of alleles

• Variations from Mendel’s Laws tells us that interpretation of phenotype/genotype relation is more complex than the traits he observed and showed

Terms that relate to phenotype, not genotype

Recessive, dominant, codominant, and incomplete dominance

Sickle Cell Anemia

• A disease where RBCs become thing and elongated. The sickled cells cannot carry oxygen efficiently, which contributes to a variety of possible symptoms

• Individuals with one HbS allele (carriers) produce both normal and sickled RBCs. This heterozygous genotype expresses both the wildtype and mutant, they are codominant.

  • Since both cells are made, oxygen is delivered to the tissues by the normal shaped blood cells

  • However, individuals with two HbS alleles develop sickle cell anemia, making this trait recessive

Two postulates are basic principles of gene transmission

• Genes are present on homologous chromosomes (gene A is on chromosome 1 for ALL individuals in a species)

• Chromosomes segregate and assort independently (1 from mom, 1 from dad)

Phenotypic characters are influences by many different genes and their products

• genetics

• interactions

• environmental factors

Gene interaciton

• Several genes influence a particular characteristic

• Cellular function of numerous gene products contributes to development of common phenotype

Deviation from the Mendelian ratio indicates:

An interaction of two or more gene products produces the phenotype

2 types of gene interactions that occur and modify Mendelian ratios

• different genes control the same trait, collectively producing a new phenotype (ex. chickens)

• One gene masks the expression of others (epistasis) and alters the phenotype (ex. labs)

Epistasis

• The effect of one gene pair masks or modifies the effect of another gene pair, but no new phenotype is produces

• Several types of epistatic interactions, each results in a modification of the F2 dihybrid ratio of 9:3:3:1

• An allele at one locus may prevent the expression of an allele at a second locus

Ways epistasis may occur

dominant and recessive

Epistatic gene

Determines whether or not a trait will be expressed (phenotype may be masked or modified). May be dominant or recessive

Hypostatic gene

The gene whose effects are masked or modified

Recessive epistasis

Phenotype is masked by two recessive alleles at the epistatic locus (ex. coat color in mice)

Dominant epistasis

Phenotype is masked by a dominant allele at the epistatic locus (ex. fruit color in summer squash)

Essential genes

Required for survival and mutations (lethal alleles) may result in death

• are often genes of key metabolic enzymes

Dominant lethal alleles

Cause death when one copy of the lethal allele is present (both homo and heterozygotes)

ex. Huntington’s disease

Recessive lethal alleles

Cause death only when two copies of the lethal allele are present. One copy causes lethality in hemizygous condition. Lethality is not always immediate.

ex. curly wing flies, Tay-Sachs disease

Does the phenotype invariably represent the genotype?

Development of a multicellular organism from a zygote is a series of generally irreversible phenotypic changes resulting from interaction of the genome and the environment. Four major processes are involved:

• Replication of genetic material

• Growth

• Differentiation of cells into types

• Arrangement of cell types into defined tissues and organs

Internal and external environments interact with the genes by controlling their expression and interacting with their products

Environments vary; therefore, the phenotype does not always correspond to the genotype

Expressivity

Degree to which a genotype is phenotypically expressed. Depends on both genetics and environment.

Its either: Constant - genotype shows expected phenotype or Variable- genotype shows a range of phenotypes.

Penetrance

Expressed as the percent of a population that expressed a trait. Phenotype depends on both the genotype and the environment.

Complete Penetrance (100%)

All organisms with the same genotype show the expected phenotype

Incomplete Penetrance (<100%)

Not all of the organisms show the expected phenotype

Some genes can have both:

Incomplete penetrance and variable expressivity (ex. neurofibromatosis autosomal dominant)

Effects of Environment (external and internal)

• Age of onset. Different genes are expressed at different times during the life cycle, and programmed activation/inactivation of genes influences many traits. (ex. male pattern baldness)

• Sex affects the expression of some autosomal genes

• Temperature effects (ex. red flowers at 23 degrees and white flowers at 18 degrees)

Sex-limited traits

Appear in one sex but not the other

Sex-influenced traits

Appear in both sexes, but the sexes show either a difference in frequency of occurrence or an altered relationship between genotype and phenotype.

Homologous chromosomes

2 copies of a chromosome, not identical. Chromosome 13, 1 copy from mom + 1 copy from dad = homologous pair. XY is not homologous, XX is. (4 chromatids = 2 chromosomes = 1 pair)

Sister chromatids

Identical chromosomes. Appear during the S phase of interphase to separate into new cells.

Euploid

Number of sets of chromosomes

Aneuploidy

Chromosome count deviates from the normal chromosome complement. Can be caused by nondisjunction or translocation.

Nullisomic

Missing a homologous pair

Monosomic

Missing a single chromosome

Trisomic

Having an extra chromosome (lethal in humans)

Tetrasomic

Having an extra homologous pair.

Nondisjunction

Failure to separate during meiosis (either I or II), causing an imbalance of chromosomes in gametes.

Deletion

loss of genetic information on a chromosome (ex. a chromosome with information A-G loses DE)

Duplication

A piece of the information on a chromosome is duplicated (ex. chromosome A-G duplicates BC). Arise from unequal crossing over between synapsed chromosomes during meiosis.

Inversion

A piece of a chromosome is flipped (ex. chromosome A-G has section B-D flipped)

Nonreciprocal Translocation

A piece of a chromosome is added to a nonhomologous chromosome without receiving DNA in return.

Reciprocal translocation

Pieces of 2 chromosomes attach to each other (nonhomologous chromosomes)

Robertsonian Translocation

q arm of chromosome 21 fused with q arm of either chromosome 14 or 15

• Only occurs between acrocentric chromosomes

• Long arms of 2 different chromosomes break off and join together

• Short arms join and are lost

Monoploidy/Polyploidy

• Changes in complete sets of chromosomes

• Result from either meiotic division without cell division or nondisjunction.

Monoploidy

One of each chromosome (no homologous pair). Normal in certain life cycle stages of sexually reproducing organisms. Rarely observes in diploid organisms

Polyploidy

More than one homologous pair of all chromosomes. All chromosomes present in 3 or more copies. Lethal in most animals, tolerated in plants.

• Even numbered polyploids are more likely to be fertile, odd numbered are usually sexually sterile

Interspecific Hybrids

Mating of two different species

Sexually fertile

Gametes have a complete set of chromosomes (viable)

Sexually sterile

Gametes do not have a complete set of chromosomes (nonviable)

Alloploidy

Polyploid offspring have a combination of chromosome sets from different species as a consquence of hybridization

Autoploidy

Polyploid offspring have multiple sets of the haploid compliment, parents are the same species