Unit 5: Heredity

Examples of Asexual Reproducion

• Mitosis

• Produces exact copies

• Single-celled eukaryotes

• Simple multicellular eukaryotes, the example below is a simple animal  animal Hydra species. It clones itself by budding.

Examples of Sexual Reproduction

• Reproduction in complex multicellular organisms

• Variation in offspring is the point

• Reproductive cells are produced by meiosis.

Outcome of Meitosis

• Produces gametes (reproductive cells)

• Two cell divisions

• Reduces the number of chromosomes by half

• The cells that are produced are different from each other and from the original cell

How can a change in ploidy be expressed in terms of n? When does it occur?

• (ploidy reduces in Metaphase 1)

  • Diploid 2n→ haploid n  (other species can be 4n - 6n!)

Draw a general diagram of meiosis

Interphase

• DNA is replicated

• The cell prepares for division

• Interphase only happens once

Prophase 1

Homologous pairs can exchange segments during crossing over (increasing variation and driving evolution faster)

Metaphase 1

• Homologous pairs line up in the middle of the cell - opposite each other (not above each other as in mitosis)

• Independent assortment

Independent assortment

Pairs randomly line up in the middle (Mendel’s 1st Law)

Anaphase 1

Homologous pairs are separated; ploidy reduced

Telophase 1

Two nuclei form and the cell splits into two

In each pair, one chromosome is from each?

Parent

The chromosomes contain genes for the same trait (ex. hair color).

Draw a diagram of crossing over

Tetrad

Name the steps of crossing over

Homologous pair, synapsis, crossing over, recombinant chromosomes

Homologous pair

These are replicated chromosomes, so each side (sister chromatid) is identical.

Recombinants

Genetic combinations that did not previously exist

Meiosis 1

Homologous pairs are separated

Meiosis 2

Sister chromatids are separated

Prophase 2

Crossing over does not occur

Metaphse 2

Chromosomes line up in the middle of the cell

Anaphase 2

Sister chromatids are separated

Telophase 2

Nuclei form and cells split (either 4 sperm cells, or 3 polar bodies and and egg)

Variation is an advantage, why?

It increases the likelihood that some members of a population will survive disease, disaster, etc. (driving natural selection)

What increases variation in a population

Sexual reproduction

Crossing over

During meiosis mixes alleles across homologous chromosomes, creating new combinations of traits on each chromosome

Independent assortment

During metaphase I of meiosis results in the formation of gametes that are different from each other (2²³ combinations in gametes)

Random fertilization

Of an egg cell (any two parents will produce a zygote with 2²³ x 2²³ possible diploid combinations)

When done correctly, meiosis should result in? What would the zygote have?

Gametes with 23 chromosomes each.

That way, the zygote has 46 chromosomes - 44 of which are autosomes and 2 of which are sex chromosomes (in humans, the X & Y).

What errors can occur in meiosis?

Nondisjunction or breaking of chromosomes

Nondisjunction

Chromosomes don’t separate properly during meiosis

Problems with the meiotic spindle cause daughter cells to have too many or too few chromosomes

Breaking of chromosomes

• Deletion

• Duplication

• Inversion

• Translocation

Nondisjunction 1

Homologous pairs do not separate properly during Meiosis 1

Nondisjunction 2

Sister chromatids do not separate during Meiosis 2

Results of nondisjunction

Zygotes with 3 copies of a chromosome (trisomy) or 1 copy (monosomy) instead of 2 copies of each chromosome

Most of the time nondisjuncition results in?

Miscarriage

What are some examples of nondisjunction when a miscarriage does not happen?

• Down Syndrome (Trisomy 21)

• Klinefelter’s Syndrome (XXY male)

• Turner’s Syndrome (XO)

• Jacob’s Syndrome (XYY)

Karyotype

Picture of chromosomes

Changes in Chromosome Structure

Deletion, duplication, inversion, translocation

Deletion

Removes a chromosomal segment

Duplication

Repeats a segment

Inversion

Reverses a segment within a chromosome

Translocation

Move a segment from one chromosome to another, nonhomologous one

Gregor Mendel

Documened the inheritance of peas

Mendel’s findings

• Traits come in alternate versions (alleles)

• For each characteristic, an organism inherits 2 alleles, 1 from each parent

• Some traits mask others (dominant masks recessive)

Phenotype

Physical appearance of a trait

eg. purple or white

Genotype

an organism’s genetic makeup

Homozygous

Same alleles; PP or pp

Heterozygous

Different alleles; Pp

Test cross

A visible, low tech test for genotype

An organism has the dominant phenotype but an unknown genotype could be?

Homozygous dominant or heterozygous)

How would the cross test go for the organism with an unknown genotype but dominant phenotype?

• Cross the organism with one that is homozygous recessive - the ‘test’

  • If some of the offspring have the recessive phenotype → organism was heterozygous

  • If none of the offspring have the recessive phenotype → organism was homozygous dominant

Mendel’s first law

Law of segregation (coming apart)

Law of segregation

• During meiosis, alleles segregate (separate)

  • Homologous chromosomes separate during anaphase I

• Each allele for a trait is packaged into a separate gamete.

Mendel’s 2nd Law

Law of independent assortment

Law of independent assortment

Different chromosomes separate into gametes independently

In this diagram, you can have multiple combinations of red and blue chromosomes in each gamete

When does independent assortment occur?

Non-homologous chromosomes align independently during metaphase I

For which genes is independent assortment true?

Only true for genes on separate chromosomes or on same chromosome but far apart so that crossing over happens frequently

Autosomal dominant disease

Huntington’s

Autosomal recessive disease

Cystic fibrosis

Draw a key for pedigrees

Pedigrees Autosomal, X-linked, dominant and recessive patterns of inheritance: provide a brief indication of each

What causes dominance vs. recessive?

Dominant and recessive alleles code for two different proteins because their nucleotide sequences are different. (Mendel’s 3rd Law)

For a gene coding an enzyme: what if it is homozygous dominant?

100% functional protein produced

For a gene coding an ezyme: what if it is heterozygous?

Only 50% functional protein produced

• 50% may enough to accomplish the cellular function

• The dominant allele may be up-regulated to compensate for the lack of function due to the recessive allele

For a gene coding an enzyme: what if it is homozygous recessive?

0% functional protein produced

Genetics & Probability

Mendel’s laws of segregation and independent assortment reflect the same laws of probability that apply to tossing coins or rolling dice.

What is the probability of passing on B in a gamete? What is this similar to?

50%

Tossing heads

Rule of Multiplication

Chance that 2 or more independent events will occur together

Provide an example of the rule of multiplication

Probability that 2 coins tossed at the same time will land heads up: ½ x ½ = ¼

Probability of Pp x Pp having offspring pp: ½ x ½ = ¼

Given the cross AABbccDdEEFf x AaBbccDdeeFf, what is the probability offspring will have the genotype AabbccDdEeFF?

1/64

Rule of Addition

Chance that an event can occur 2 or more different ways

Probability of Bb x Bb → Bb?

1/2

Chi square

An analysis to determine if the results of a cross fit the expected ratio.

Outcomes of a Chi Square

• If calculated value > critical value → reject null hypothesis

• If calculated value < critical value → accept null hypothesis

Why might you reject the null hypothesis?

• You have a small sample size.

• Your sample is not representative of the larger sample.

• The genes do not show independent assortment  - non-mendelian genetics.

  • May be linked (close together on the same chromosome)

  • May be sex-linked (on one of the sex chromosomes)

Chi Square formula

Degrees of freedom

Number of phenotypes - 1

Mendel worked with a simple system following what principles?

• Peas are genetically simple

• Most traits are controlled by a single gene

• Each gene has only 2 alleles, 1 of which is completely dominant to the other

What did Mendel not consider?

The relationship between genotype and phenotype is rarely that simple:

Mnemonic for dihybrid crosses

FOIL (first, outside, inside, last)

Incomplete dominance

Heterozygotes show an intermediate phenotype

Example of incomplete dominance

• RR = red flowers

• Rr = pink flowers

• rr = white flowers

Null hypothesis

No relationship between two variables; the finding probably occured by chance

Alternative hypothesis

States the opposite of a null; there is a relationship between two varibales; the finding did not occur by chance

Codominance

2 alleles affect the phenotype in separate, distinguishabl

Example of codomniance

ABO blood types - A and B are codominant, so people with type AB blood have A and B antigens on their blood cells

Blood compatibility

Matching compatible blood groups is important for blood transfusions

What in a person’s blood affects blood compatibility?

A person produces antibodies in their blood plasma against the antigens on foreign blood cells

Pleiotropy

Most genes are pleiotropic - affect more than one phenotypic character

• Wide-ranging effects due to a single gene

Examples of pleiotropy

• Ex. dwarfism (achondroplasia)

• Ex. gigantism (acromegaly)

Epistasis

One gene masks another

Example of epistasis :)

• Ex. coat color in mice 

  • Pigment (C) is dominant to no pigment (c)

  • Black pigment (B) is dominant to brown pigment (b)

  • cc is albino, regardless of the B allele

• Ex. coat color in Labrador retrievers 

  • Pigment (E) or no pigment (e)

  • Black pigment (B) or brown pigment (b)

Polygenic inheritance

Some phenotypes are determined by the additive effects of 2 or more genes on a single character

Examples of polygenic inheritance

Many human traits  - skin color, height, eye color, etc.

Nature vs. Nurture (Phenotypic Plasticity)

Phenotype is controlled by the environment and genes

Examples of phenotypic plasticity

• Ex. coat color in Himalayan rabbits influenced by heat sensitive alleles

• Ex. color of Hydrangea flowers influenced by soil pH

• Ex. human skin color influenced by UV radiation

Sex-linked traits

Human sex chromosomes: X and Y

Female

2X chromosomes

Male

XY