Chapter 11

Intro. to Genetics

Chap. 11-4 Meiosis

• Mitosis – division of body cells (somatic cells)

• End result - 2 cells identical to starting cell w/same number of chromosomes

• Meiosis – production of sex cells (sperm & egg) (gametes)

• End result – 4 cells with ½ the chromosome number as starting cell

Karyotype

• A picture of one’schromosomes arranged in pairs

• Each pair are alikein that they carrygenes for the sametraits

• Are called homologous chromosomes

Homologous Pair

Chromosome Number

• Gametes – called haploid b/c they contain only half the number of chromosomes

• Called the n number (n refers to number of chromosomes in gametes)

• Gametes have only one member of each homologous pair

Chromosome Number

• Somatic cells – called diploid b/c they contain two of each chromosome or 23 pairs (in humans)

• Each pair – called a homologous pair

• Because each pair forms a4-stranded group, they are also called a tetrad

• Come together in Prophase I in a process called synapsis

Divisions of Meiosis

• Meiosis occurs in 2 major divisions:

• 1st division – Meiosis I - separates homologous pairs of chromosomes (reduction phase)

• 2nd division – Meiosis II - separates chromatids of each chromosome (division phase) & is similar to mitosis

Meiosis

Meiosis

• Important events to remember:

• Synapsis & crossing over – Prophase I

• Separation of homologous chromosomes – Anaphase I

• Division of chromatids – Anaphase II

Importance of Meiosis

• Causes variation in chromosomes by:

• 1. Crossing over - when homologous chromosomes

exchange genes

Importance of Meiosis

• 2. Usually results in random segregation (separation) of chromo-somes

Both result in newgenetic combin-ations in the off-spring

Mitosis vs. Meiosis

Mitosis Meiosis

1 division 2 divisions

Produces 2 genetically identical cells

Produces 4 genetically different cells

Cells are diploid Cells are haploid

Produces body (somatic) cells

Produces sex cells (gametes)

Meiosis in Males

Meiosis in Females

CHAP. 11-1 & 11-2 Mendel’s Laws of Heredity

• Genetics – study of heredity

• Heredity – passing of traits from parent to offspring

• Traits transmitted by genes located on chromosomes

• Most traits determined by a pair of genes located on paired chromosomes (homologous)

Mendel’s Laws of Heredity

• Gregor Mendel – “Father of Genetics”

• Studied garden pea plants; 2 reasons:

1.Have 7 contrast-ing traits

• ex. – tall vs. short, green pod vs. yellow, etc.

Mendel

• 2. Easy to cross-pollinate -

(the transfer of male pollen

grains from one plant to the

female organ of another plant)

(usually pea plants self-

pollinate

Genetics Vocab.

• Genotype – genetic makeup of an organism (Tt)

• Phenotype – physical makeup of an organism (tall)

• Homozygous (pure) – when 2 genes for a trait are the same (TT or tt)

• Heterozygous (hybrid) – when 2 genes for a trait are different (Tt)

• Monohybrid cross – one with only 1 trait crossed

• Alleles – different forms of a gene (T or t)

Mendel’s Experiments

• Mendel first crossed pure tall plant with pure short plant

• To work a genetics problem:

• Step 1- Pick symbols:

• T - tall, t - short

• Step 2 - Diagram the cross:

• TT x tt (Parents or P generation)

Mendel’s Experiments

• Step 3 – Do the Punnett square:• TT x tt

• offspring are called 1st filial or F1 generation)

• one trait shows up (tallness) & one disappears (shortness)

Mendel’s Experiments

• Step 4 – List ratios:• Genotypic ratio –

• Phenotypic ratio -

Mendel’s Experiments

• Next Mendel did a second generation cross (cross 2 plants from the 1st generation)

• F2 cross = Tt x Tt

• Now the tall trait appears

• Offspring are called F2 generation

Monohybrid Crosses

Mendel’s Laws

• Law of Dominance – in an

organism w/contrasting alleles

(different forms of a gene), one

gene shows up & the other disappears • Gene that shows up – dominant• Gene that disappears – recessive• Law of Segregation – pairs of genes separate

when gametes are formed in meiosis (shown in the Punnett square

Ch. 11-3 Mendel’s Laws

• Law of Independent Assortment – pairs of genes separate independently of each other

• Dihybrid cross – one with 2 different traits

Dihybrid Cross – used to show Mendel’s Law of Independent Assortment

•F-o-i-l (first – outside – inside - last)

In guinea pigs, black fur is dominant over white. Rough coat is also dominant over smooth. Cross a homozygous black, rough guinea pig with a heterozygous black, rough guinea pig.

Incomplete Dominance

• Incomplete dominance – when 1 gene is not completely dominant over the other; a blending of traits occurs

• Problem - In some flowers, red is incompletely dominant over white. The heterozygous condition produces pink flowers.

• Pick symbols: R – red, R′ - white• Possible genotypes:• RR- red• R′R′ - white• RR′ - pink

Codominance

• Codominance – when neither gene is dominant; both genes in a pair are expressed

• In cattle, red hair is codominant with white. The heterozygous condition produces a roan (mix of red & white) offspring.

• Pick symbols – R-red; W-white R• Possible genotypes:• RR – red• WW – white• RW - roan

Bio. Bingo• Mendel alleles gametes• Independent assortment segregation hyrbid• Heterozygous F2 fertilization• Genotype self-pollination

probabilityb• Phenotype dominant• Genetics monohybrid• Heredity incomplete dominance• Recessive codominance• 1st filial 1:2:1• Homozygous cross pollination• Peas dominance• Traits dihybrid• Genes dominant• Punnett square 3:1