Chapter 10

Mendel & Meiosis

10.1: Mendel’s Laws of Heredity

Heredity- passing on of characteristics from parent to offspring

Traits- characteristics that are inheritedGenetics- branch of biology that studies

heredity

Gregor Mendel

Austrian monk, 1860’sCarried out thousands of genetic

experiments with garden peas

Why peas?

1. Peas reproduce sexually Form gametes- sperm & eggs Fertilization unites these gametes to form

a zygote which develops into a seed

2. Peas normally self-pollinate or fertilizeMendel could cross-pollinate them

manually & be sure of the parents in a given cross

3. Peas are easy to grow & reproduce quickly

Pea plant traits

Pea plants have many distinct traits that are either/or characteristics

Seed shape- round or wrinkledSeed color- yellow or greenFlower color- purple or whitePlant height- tall or shortEtc.

Hybrids

Hybrid- offpsring of parents that have different forms of a trait

Mendel crossed parents with different traits and recorded data on what the resulting hybrid offspring were like

What Mendel discovered:

1. The rule of unit factors- each organism has 2 factors that control each of its traits

Alleles- alternate forms of a gene Dominant allele- represented by capital

letter (A, B, C…) Recessive allele- represented by

lowercase letter (a, b, c…)

2. The rule of dominance- between alternate forms of a gene, one will be dominant over the other- the dominant form will show up when paired with the recessive form

3. The law of segregationEvery individual has two alleles for each

geneWhen gametes are produced, each

gamete receives only one of these alleles

More vocabulary:

Phenotype- the way an organism looksGenotype- the alleles an organism has

(the letters)Homozygote (homozygous)- an organism

with two alleles that are identical (AA, aa)Heterozygote (heterozygote)- an organism

with two alleles that are different from one another (Aa, Bb, Dd)

Let’s imagine….

We have a pea plant with purple flowers.Purple (F) is dominant over white (f)The plant’s genotype could be homozyote,

______, or heterozygote, ______.The plant’s phenotype is ________.

If we have a pea plant with white flowers, the plant’s genotype must be ________.

It is a _____________.The plant’s phenotype is _________.

Gametes

Diploid cells- cells whose nuclei contain two sets of homologous chromosomes

Diploid number- total number of chromosomes in a diploid cell

Abbreviated 2n46 in humans, 44 autosomes & 2 sex

chromosomes

Haploid cells- cells with a single set of chromosomes (1 of each homologous pair)

Gametes- sperm or eggsHaploid number- total number of

chromosomes in a haploid cellAbbreviated n23 in humans, 22 autosomes & 1 sex

chromosome

Fertilization of egg (n) by sperm (n) results in a fertilized egg called a zygote (2n)

Gametes are the result of a division process called meiosis

Meiosis is the process that separates the letters in a genotype into separate genotypes

Meiosis

Meiosis is a reduction division- the number of chromosomes is reduced in the division

Occurs in reproductive organs- ovaries & testes

Involves two divisionsFour daughter cells resultEach daughter cell has only ½ as many

chromosomes as the starting cell

Punnett Squares

Tool used by geneticists to predict expected results of crosses between parents of known genotypes.

Monohybrid cross- cross between parents that differ in one trait

Punnett squares determine the probability that a certain genotype will occur in the offspring of a certain cross

10.2 Meiosis

In mitosis, each new cell produced has the exact number of chromosomes as the original cell

Meiosis- the making of sperm or eggs

Homologous Chromosomes

Somatic cell- typical body cell46 chromosomes that come in 23

matched pairsHomologous chromosomes-

chromosomes in a matched pair that have genes controlling the same inherited characteristics

Locus- location of a particular gene on a chromosome

Homologous chromosomes have genes for the same trait at the same locus, but they may have different versions of that gene

Types of Chromosomes

1. Autosomes Found in both males and females 22 pairs

2. Sex chromosomes Determine gender 1 pair- XX or XY Only small parts are homologous, but they

behave like homologous pairs during meiosis

We inherit one chromosome from each pair from our mother and one from our father.

Meiosis I: Homologous Chromosomes Separate

InterphaseChromosomes duplicateProphase ISynapsis- homologous chromosomes (each

composed of 2 sister chromatids) come together as pairs to forma tetrad

Crossing over- legs of homologous chromosomes flop over each other and may exchange pieces- source of genetic variation

Spindle forms

Metaphase ITetrads line up on metaphase plate

Anaphase IOne homologous chromosome from each

tetrad moves to opposite poleOnly tetrads split up, not sister chromatids

Telophase I and cytokinesisEach pole has haploid set of

chromosomes, one from each homologous pair

Cytokinesis occurs

Interphase again?

In some species, interphase occurs between Meiosis I and II

Chromosomes uncoilNuclear membranes reformNo chromosomal duplication

Meiosis II: Sister chromatids separateProphase IISpindle formsMetaphase IIChromosomes line up on metaphase plateAnaphase IISister chromatids are pulled apartTelophase IINew nuclear membranes formCytokinesis occurs

Results of Meiosis

Four new haploid daughter cellsIn males- four viable spermIn females- one viable egg, three polar

bodies

Oogenesis