Chapter 10 Mendel & Meiosis

10.1: Mendel’s Laws of Heredity Heredity- passing on of characteristics from parent to offspring Traits- characteristics that are inherited Genetics- branch of biology that studies heredity

Gregor Mendel Austrian monk, 1860’s Carried out thousands of genetic experiments with garden peas

Why peas? 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 fertilize Mendel 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 wrinkled Seed color- yellow or green Flower color- purple or white Plant height- tall or short Etc.

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: 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 segregation Every individual has two alleles for each gene When gametes are produced, each gamete receives only one of these alleles

More vocabulary: Phenotype- the way an organism looks Genotype- 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 2n 46 in humans, 44 autosomes & 2 sex chromosomes

Haploid cells- cells with a single set of chromosomes (1 of each homologous pair) Gametes- sperm or eggs Haploid number- total number of chromosomes in a haploid cell Abbreviated n 23 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 divisions Four daughter cells result Each 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 cell 46 chromosomes that come in 23 matched pairs Homologous 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 Interphase Chromosomes duplicate Prophase I Synapsis- 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 I Tetrads line up on metaphase plate Anaphase I One homologous chromosome from each tetrad moves to opposite pole Only tetrads split up, not sister chromatids

Telophase I and cytokinesis Each 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 uncoil Nuclear membranes reform No chromosomal duplication

Meiosis II: Sister chromatids separate Prophase II Spindle forms Metaphase II Chromosomes line up on metaphase plate Anaphase II Sister chromatids are pulled apart Telophase II New nuclear membranes form Cytokinesis occurs

Results of Meiosis Four new haploid daughter cells In males- four viable sperm In females- one viable egg, three polar bodies

Oogenesis