Unit 5 AP Biology

Meiosis

Creates haploid gametes for reproduction

Haploid gametes join to form a diploid cell so the chromosome number is restored and maintained through generations

Meiosis I

Separates homologous chromosomes

Happens after interphase

Results in two daughter cells

Prophase I

Synapsis and crossing over

Synapsis

Homologous chromosomes pair up and connect to eachother, forming a tetrad.

• Recombination occurs at the chiasmata (physical connection point of two non sis chromatids), matching genes at the same location are swapped.

• 2 recombinant chromatids are formed.

Metaphase I

Homologous pairs line up at the metaphase plate

Anaphase I

Homologous pairs separate

This creates 2 unique daughter cells with haploid numbers of chromosomes

Meiosis II

Separates sister chromatids

Reduces the DNA content from 2 copies back to 1.

DNA replication does not occur before

Results in 4 haploid, unique cells

Anaphase II

Sister chromatids are pulled apart.

The result is four unique haploid cells.

Nondisjunction

Failure of sister chromatids or homologous chromosomes to separate properly =, leading to abnormal numbers of chromosomes in resulting cells.

Nondisjunction in Meiosis 1 vs meiosis 2

Meiosis 1: homologous chromosomes fail to separate. All resulting gametes abnormal. Two of the final four gametes will be missing a chromosome, and the other two will have both homologous chromosomes.

Meiosis 2: sister chromatids will fail to separate. 50% of gametes are normal. One of the two remaining gametes will have an extra chromosome. One will be missing a chromosome.

Genetic diversity with meiosis

1) Crossing over—produces recombinant chromosomes with a mix of parental genes

2) Independent assortment—Homologous pairs line up randomly at the metaphase plate, creating a mix of maternal vs. paternal chromosomes in cells.

3) Random fertalization

Homologous chromosomes

Pair of chromosomes that are the same size and carry the same genes at the same locations.

Universal genetic code

With minor exceptions, the same codons code for the same amino acids across all organisms

Genes

Segments of DNA that code for proteins and functional products

Alleles

A specific version/variation of a gene

P generation

Parental generation, true breeding

• Organism that is homozygous for a specific trait and consistently passes it down.

F1 generation

Hybrid offspring

F2 offspring

Offspring of the F1 generation

Recessive allele

A version of a gene that expresses its phenotype when an individual has 2 copies of it

Dominant allele

A version of a gene that expresses its phenotype even when 1 copy is present

Homozygous

Has two identical alleles for a gene

Heterozygous

Has two different alleles for a gene

Genotype

Specific combination of alleles for a specific gene

Phenotype

Observable characteristics/traits that are a result of the genotype + environment

Testcross

An unknown genotype with a dominant phenotype is crossed with a homozygous recessive.

• Homo dom: all dominant phenotype offspring.

• Hetero: 1:1 ratio

Mendels laws

Law of dominance: in a heterozygous individual one allele (dominant) will mask the other (recessive) effect on phenotype

Law of segregation

Law of independent assortment

Law of segregation

Two alleles for a gene will separate during gamete formation.

• Each gamete will receive one allele per gene

Monohybrid cross

Tracks the inheritance of one trait.

• Typically, two hybrids are crossed.

3:1 phenotypic ratio

Law of independent assortment

Alleles of different genes are distributed into gametes independently of one another.

• Inheritance of one trait does not influence the inheritance of the other.

The alignment of one homologous chromosome pair at the metaphase plate does not influence alignment of another.

Exception: linked genes.

Dihybrid cross

Tracks the inheritance of two traits

• Typically, two hybrids are crossed (heterozygous for both traits)

9:3:3:1 phenotypic ratio

Multiplication rule

Use for independent events

• Occurrence of one event does not effect the occurrence of the other event

AND

Addition rule

Use for mutually exclusive events

• Two events cannot happen at the same time

OR

Incomplete dominance

Neither allele is fully dominant over the other in the heterozygous individual so the phenotype of the F1 generation is a intermediate of the two phenotypes in the P generation

Codominance

Two different alleles are equally expressed in the heterozygous individual

F1 phenotype has both the phenotypes

Multiple alleles

A gene has more than two possible alleles

Lethal alleles

Mutations in essential genes that cause death of the organism

Recessive: two copies needed to cause death

Dominant: only one is needed

Conditional lethal alleles

Alleles that cause lethality under certain environmental conditions

Epistasis

One gene masks, suppresses or modifies the expression of another gene.

• Contribution of one allele to the phenotype is masked/modified by another gene

Dominant: one dominant allele is needed for epistatic gene to mask 12:3:1

Recessive: two alleles needed for epistatic gene needed to mask 9:3:4

Polygenic inheritance

A trait is controlled by two or more genes

• Each gene adds an effect to the trait

• Leads to a bell shaped distribution: continuous variation in the population

Pleiotropy

One gene influences multiple traits

Sex linked traits

Traits coded for by genes on sex chromosomes

X linked recessive

Two copies needed to affect phenotype in females

Only 1 needed in males

X linked dominant

Only one needed to affect phenotype in both genders

Y linked

Genes located on the Y chromosome, only in males, only male to male inheritance

Non nuclear inheritance

Transmutation of genes found in the mitochondria or in the chloroplast

• Randomly assorted into daughter cells, so traits coded for genes in mitochondria or chloroplast do not follow Mendelian rules

Mitochondrial and chloroplast inheritance

In animals, the mitochondria is transmitted by the egg so these traits are maternally inherited.

In plants, the chloroplast are transmitted by the ovule so these traits are maternally inherited

Linked genes

Genes located close together on the same chromosome whose alleles tend to be inherited as a unit.

Parental offspring

Have allele combinations identical to one of the parents

• Parental phenotype

• Parental phenotypes of offspring higher than 50% for linked genes

Recombinant offspring

Have a new combination of alleles not present in either parent

• Offspring phenotypes that are different from the parents

Recombination frequency

Percentage of offspring showing new combinations of traits (recombinant offspring)

• Frequency of crossing over between two linked genes.

• The further apart two genes are on the same chromosome, the more likely they are to cross over

• 1 map unit is 1% RF.

• RF of 50% means the genes are far away on same chromosome or on a different one

• Maxes out at 50 because crossing over involves only 2 of the 4 chromatids so at least 50% of gametes remain parental

Phenotypic plasticity

Ability of a single genotype to produce multiple phenotypes in response to varying environmental conditions

• Lets organisms adjust to their environmental conditions

Environmental affect on phenotype

The environment can influence gene expression and protein activity, which leads to different phenotypes (phenotypic plasticity)

X inactivation

One of the two X chromosomes in each cell is randomly inactivated in early development of a female

• Ensures women have the same amount of x linked products that males do

Forms a Barr Body: inactive X chromosome found in somatic cells of female mammals

Polyploidy

A condition where a cell contains more than two complete sets of chromosomes

3n, 4n, 5n …

Monosomy

2n-1, loss of one chromosome from a pair (the cell only has 1 chromosome from the pair)

Trisomy

2n+1, the presence of an extra chromosome in a pair (the cell has 3 copies instead of the usual 2)