The chromosome basis of inheritance 

Mendel’s heredity factors

Genes

The location of a particular gene can seen by…

Putting fluorescent dye on an isolated chromosome to highlight the gene

What are chromosomes made up of

Chromosomes are made up of histonproteins and DNA

The chromosome theory of inheritance

1. Mendelian genes have specific loci (positions) on chromosomes 2. Chromosomes undergo segregation and independent assortment (meiosis)

Law of segregation

2 alleles for each gene separate during gamete formation

Law of independent assortment

Alleles of genes on non homologous chromosomes assort independently during gamete formation

What did Morgan’s experiments with fruit flies prove

Thomas hunt Morgan’s experiments with fruit flies proved that chromosomes are the location of Mendel’s heritable factors

3 parts of the cell theory

1. Cells came from other cells 2. Cells are the basic unit of life 3. All living things are made of cells

Characteristics that make fruit flies a convenient organism for genetic studies

1) They breed at a high rate 2) a generation can be bred every 2 weeks 3) they only have 4 pairs of chromosomes (8 chromosomes)

Wild type

Most common or normal phenotypes

Mutant phenotype

Abnormal phenotypes

What did Morgan mate

Male flies with white eyes with female flies with red eyes

Thomas hunt Morgan’s results from his experiment with the flies

• the F1 generation all had red eyes

• The F2 generation had a 3:1 red:white ratio (Mendel’s ratio) but only males had white eyes

• The white eyed mutant allele must be located on the X chromosomes

• Morgan’s findings supported the chromosome theory of inheritance

What does it mean if there is a plus next to the phenotype variable?

it’s the wild type phenotype

What part of the Y chromosome are homologous with the X chromosome

Only the ends

SRY gene

The sry gene are on the Y chromosome and code for the development of testes

A sperm may contains either an…

X or a y

Who has more genetic information

females

do sex chromosomes only have genes related to sex?

No

Sex-linked gene

A gene located on either sex chromosome

What does sex linked usually mean for humans

A gene on the larger X chromosome

Disorders caused by recessive alleles on the X chromosomes

Color blindness, duchenne muscular dystrophy, hemophilia, and baldness

What does the inactivated X chromosome in the cell during embryonic development condense to?

Barr body

What is a Barr body similar to?

A co-dominant phenotype

Linked genes

Genes located on the same chromosome that tend to be inherited together because they are located near each other

Testcross

Crossing someone who is homozygous recessive with a heterozygous

Parental phenotypes

Observable traits inherited by offspring that relate to one or both of the parents traits. Genes usually inherited together in specific combinations

Genetic recombination

The production of offspring with combinations of traits differing from each parent

Offspring with nonparental phenotypes (new combinations of traits)

Recombinant types or recombinants

Crossing over

When you break the physical connection between genes on the same chromosome

The more farther apart genes are on a chromosome…

The more likely crossing over will happen

recombination frequency

Recombinants/total offspring

Recombination frequency

Tells you how likely a crossing over event will happen between two genes

Genetic map

An ordered list of the genetic loci along a particular chromosome.

The higher the probability that a crossover event will occur…

the higher the recombination frequency

Linkage map

A genetic map of a chromosome based on recombination frequencies.

Map units

Distances between genes, one map unit, or centimorgan, represent a 1% recombination frequency. Map units indicate relative distance and order, not the precise location of genes. Formula: Recombination Frequency/2

Genes far apart on the same chromosome can have a recombination frequency near…

50%

How did geneticists develop cytogenetic maps of chromosomes?

By using methods like chromosomal banding.

Cytogenetic map

Indicates the positions of genes with respect to chromosomal features.

Lethal dominant

Dwarfism

Nondisjunction

Pairs of homologous chromosomes do not separate normally when they meet during meiosis. Gametes acquire an abnormal amount of chromosomes.

Which is worse to have nondisjunction in?

Nondisjunction of homologous chromosomes in Meiosis 1 because all the gametes are impacted (2 are n+1 and 2 are n-1). In meiosis 2, the sister chromatids are split like this: one is n+1, one is n-1, and the last two are n.

How many sperms and egg do males and females get after a round of meiosis?

A male gets 4 sperm and a female gets 1 egg after a round of meiosis.

Down syndrome

3 pairs in the 21st chromosome (3/21).

Aneuploidy

Results from the fertilization of gametes in which nondisjunction occurred. More disruptive to homeostasis because their are different amount of chromosomes everywhere.

Monosomic zygote

Has only 1 copy of a particular chromosome (Barr Body - Turners syndrome)

Trisomic zygote

3 copies of a particular chromosome (Down syndrome and Klinefelter syndrome). Human: 2n+1=47

Polyploidy

An organism has more than 2 complete sets of chromosomes. (For a higher crop yield in GMOs). Multiple sets of DNA and impact every pair, but not as bad as aneuploidy. Tetraploidy - 4 sets of chromosomes. Polyploidy is common in plants, but not animals. Polyploids are more normal in appearance than aueuploids.

Crossing over cannot occur if what happens?

When there is a breakage of a chromosome.

Deletion

Removes a chromosome segment, lose many genes.

Duplication

Repeats a chromosomal segment, duplicating many genes.

Inversion

Reverses a chromosomal segment

Translocation

Deletes a segment from 1 chromosome and adds it to another, most disruptive because 2 chromosomes are impacted.

Why do people with down syndrome all look the same?

They all have the same anueploidy (47 chromosomes).

Autosomal aneuploidy

Extra or one less chromosome on one of the autosomatic chromosomes pairs.

Klinefelter syndrome

The result of an extra chromosome: in a male, XXY and in a female, XXX. Sterile (cannot make kids).

SRY Gene

The gene in males that makes testes (SRY you are a guy).

Turner syndrome (Monosomy X)

When a female has 1 X chromosome, sterile (cannot make kids), and only known viable monosomy in humans.

Why are humans able to only have one X chromosome?

Men have only 1 X chromosome and females have Barr Bodies.

Cri du chat (cry of the cat)

Results from a specific deletion in chromosome 5. A child born with this syndrome is mentally retarded and has a catlike cry, individuals usually die in infancy or early childhood.

Chronic myelogenous leukemia (CML)

A cancer caused by translocations of chromosomes.

2 normal exceptions to Mendelian genetics

Genes located in the nucleus and genes located outside the nucleus.

Genomic imprinting

Variation in phenotype, increases genetic information.

Imprinting is the result of…

Methylation (adition of CH3 and non-polar) of DNA.

Extranuclear genes or cytoplasmic genes

Genes found in organelles in the cytoplasm.

What do mitochondria, chloroplasts, and other plant plastids carry?

Small circular DNA molecules (bacteria/prokaryotes)

Why are extranuclear genes inherited maternally?

The zygotes cytoplasm comes from the egg.

Where did the first evidence of extranuclear genes come from

Studies on inheritance of yellow or white patches in leaves on a green plant.