Genetics Test #3 - Variation in Chromosome structure, mendelian inheritance, pedigrees/probabilities, and extensions of mendelian inheritance

Genetic Variation

Differences between members of the same species or those of different species

Karyotype

micrograph of metaphase chromosome

Ways to identify chromosomes in a karyotype

Binding Pattern, Size, and Centromere

What are the four centromere positions?

Submetacentric, metacentric, acrocentric, and telocentric

What are the longer and shorter arms of chromosomes called?

Shorter arm → p

Longer arm → q

How can you tell the difference between two chromosomes that are the same size?

Look at banding patterns

How can chromosome structure be altered?

• Change in total amount of genetic material

• Rearrangement of genetic material

How can genetic material be altered?

Deletion and Duplication

How can genetic material be rearranged?

Inversions and translocations

What is an interstitial deletion?

An internal deletion within and chromosome

What is a terminal deletion?

An end deletion of a chromosome

Cri-du-chat is an example of what type of deletion?

Terminal deletion - loss of part of chromosome 5

Prader-Willi and Angelman Syndrome is what type of deletion?

Interstitial deletion - loss of part of chromosome 15

Williams Syndrome is an example of what type of deletion?

Interstitial deletion

A chromosome with an ———- has a segment that has been flipped to the opposite orientation.

inversion

What is a pericentric inversion?

Centromere is included

What is a paracentric inversion?

Inversion without centromere

What is position effect?

A gene is repositioned in a way that alters its gene expression?

• Ex. euchromatin to heterochromatin area

Individuals with one copy of a normal chromosome and one copy of an inverted chromosome

Inversion Heterozygotes

Character

The morphological characteristics of an organism

• eg. eye color is a character

Trait

describes the specific properties of a character

• eg. blue eyes is a trait

Mendel’s Law of Segregation

Paired factors for a given character segregate randomly so that half of the gametes receive one factor and half of the gametes receive the other

The Law of Independent Assortment

The segregation of any pair of hereditary determinants is independent of the segregation of other pairs

Involved in crossing two variants of a singular characteristic

Monohybrid cross

Crossing individual plants that differ in two characters and observing both

Dihybrid cross.

Two copies of a gene separate from each other during transmission from parent to offspring

Modern interpretation of Mendel’s Law of Segregation

How can independent assortment be proved?

Through a dihybrid test-cross

Dihybrid test cross

a cross with homozygous recessive for both genes

Probability

the chance that the event will occur in the future

What determines whether product rule is used or binomial expansion?

For product rule, the order of events are specified. With binomial expansion, the order is not specified.

Binomial Expansion Formula

P = probability that the unordered outcome will occur

n = total number of events

x = number of events in one category

p = individual property of x

q = individual probability of the other category

Incomplete Penetrance

a dominant allele does not influence the outcome of a trait in a heterozygote individual

• example: Polydactyly

Penetrance

How many individuals in a population carry a particular allele of a gene that also express the associated trait

The degree to which a trait is expressed (how severe)

Expressivity

How could a dominant negative mutation affect an entire protein complex?

The presence of a dominant negative allele could interfere with the wildtypes in the protein to function

Incomplete Dominance

• Heterozygote intermediate between the homozygotes

  • Example: Red, White, and Pink Flowers

Overdominance (heterozygous advantage)

• The heterozygote form has a biological advantage that the homozygous species don’t express

  • Example: Sickle cell

    • HH - unaffected, not malaria resistant

    • Hh - unaffected, malaria resistant

    • hh - sickle cell disease

Temperature sensitive alleles

• Environmental conditions have an impact on the phenotype of the organisms

  • Example: Fox has brown coat in the summer and white coat in the winter

Codominance

• Neither trait is dominant to the other

  • Example: ABO blood group

Sex-Linked Traits

Traits are governed by genes on the sex chromosomes

• Color blindness, hemophilia, muscular dystrophy

Sex influenced traits

Different in males than females

• not on sex chromosomes

• difference in heterozygotes

• male pattern baldness, scurs in cattle

What are the results of a pericentric inversion?

• One normal chromosome

• One chromosome with an inversion

• Two chromosomes with a deletion and a duplication

What are the products of a paracentric inversion?

• One normal chromosome

• One chromosome with an inversion

• Two chromosomes with a deletion

  • Dicentric chromosome

  • Acentric chromosome

Haploinsufficiency

the requirement for two wildtype copies of a gene for a normal phenotypeS

Simple or Unbalanced Translocation

Transfer of genetic material occurs in only one direction

Describe Familial Down Syndrome

Majority of chromosome 21 is attached to chromosome 14

• Example of Robertsonian translocation

Reciprocal translocation (balanced translocations)

A rearrangement of genetic material, but no change in the total amount of DNA

Individuals with what type of translocation are more likely to produce gametes with unbalanced combinations of chromosomes?

Balanced translocations

For translocated chromosomes to synapse properly, a ———- ——— must form.

Translocation cross

Who is more likely to be affected by an X-linked trait?

Men, they are hemizygous - have one chromosome

Sex influenced Traits

Allele is dominant in one sex but recessive in the opposite sex

• Alleles are on autosomes not sex chromosomes

• Sex-influenced traits are visible in heterozygotes

  • Example: Scurs

    • SS - Scurs in males and females

    • ss - no scurs in males and females

    • Ss - - scurs in men and no scurs in females

  • Example: Patterned Baldness

Sex Limited Inheritance

Traits that occur in only one of the sexes

• Genes are controlled by sex hormones

  • Example:

    • Ovary development is limited to females

    • Testes growth is limited to males

    • Male plumage in birds

Lethal Alleles

• Allele is one that has the potential to cause the death of an organism

  • Example:

    • Agouti Gene in Mice

      • AA - agouti coat

      • AAy - Yellow Coat

      • AyAy - dead

Conditional Lethal Alleles

May kill an organism only when certain environmental conditions prevail (eg. temperature sensitive)

Complementation

Same phenotype caused by mutations in different genes. Wild-type offspring are produced from a cross between parents that both display the same recessive phenotype

Epistasis

a gene can mask the phenotypic effects of another gene

Gene Modifier Effect

Alleles of one gene modifies the phenotypic outcome of alleles of a different gene

• Example:

  • B dominant - Black Coat

  • b recessive - brown coat

  • D dominant - intense color

  • d recessive - diluted expression of the color

Gene Redundancy

Existence of multiple genes that perform the same function

• If one gene is missing, a paralog may be able to carry out the missing function

  • Example:

    • Inheritance of capsule shape in shepherd’s purse, gene redundancy

    • 15:1 ratio

      • TTVV/TtVv vs ttvv

Describe Alternate Segregation

• Produces viable gametes

• Chromosomes diagonal to each within the translocations cross segregate into the same cell following meiosis I

• One cell receives 2 normal chromosomes and the other receives 2 translocated chromosomes

Describe Adjacent-1 Segregation

• Four genetically unbalanced gametes

• Adjacent non-homologous chromosomes segregate into the same cell after meiosis I

• Both cells have one normal and one translocated chromosome

Describe Adjacent-2 Segregation

• Four genetically unbalanced gametes

• Centromeres do not segregate properly during meiosis I

• One cell receives both copies 1 and the other copies of the centromere on chromosome 2

Aneuploidy

• Variation in the number of particular chromosomes within a set 2n-1, 2n+1

Euploidy

- Variation in the number of complete sets of chromosome 3n, 4n, 5n

Down’s Syndrome, Triple X and Turner’s Syndrome are examples of

• Aneuploidy

• Euploidy

AneuploidyWhat

What happens to extra X chromosomes?

The are inactivated and become bar bodies

If the extra X chromosome is inactivated, why do we still observe phenotypic effects?

After fertilization, the X chromosome is still expressed before being deactivated.

Nondisjunction

refers to the failure of chromosomes to segregate properly during anaphase

Nondisjunction in Meiosis I

• Homologs do not separate

• All four gametes are abnormal

  • Two trisomic gametes

  • Two Monosomic gametes

Nondisjunction in Meiosis II

Sister chromatids do not separate and migrate to one pole

• Two abnormal gametes

  • Trisomic

  • Monosomic

• Two normal gametes