Meiosis and Mendel

Chapter 6.1-6.3

KEY CONCEPT Gametes have half the number of chromosomes that body cells have.

You have body cells and gametes.

• Body cells are also called somatic cells.• Germ cells develop into gametes.

– Germ cells are located in the ovaries and testes.– Gametes are sex cells: egg and sperm.– Gametes have DNA that can be passed to offspring.

body cells sex cells (sperm) sex cells (egg)

• Your body cells have 23 pairs of chromosomes.– Homologous pairs of

chromosomes have the same structure.

– For each homologous pair, one chromosome comes from each parent.

• Chromosome pairs 1-22 are autosomes.

• Sex chromosomes, X and Y, determine gender in mammals.

Your cells have autosomes and sex chromosomes.

Body cells are diploid; gametes are haploid.

• Fertilization between egg and sperm occurs in sexual reproduction.

• Diploid (2n) cells have two copies of every chromosome.– Body cells are diploid.– Half the chromosomes come from each parent.

• Haploid (n) cells have one copy of every chromosome.

– Gametes are haploid.– Gametes have 22 autosomes and 1 sex chromosome.

• Chromosome number must be maintained in animals.

• Many plants have more than two copies of each chromosome.• Mitosis and meiosis are types of nuclear division that make different

types of cells.• Mitosis makes

more diploid cells.

• Meiosis makes haploid cells from diploid cells.

– Meiosis occurs in sex cells. – Meiosis produces gametes.

• Meiosis differs from mitosis in significant ways.

– Meiosis has two cell divisions while mitosis has one.– In mitosis, homologous chromosomes never pair up.– Meiosis results in haploid cells; mitosis results in diploid cells.

What is heredity?• The passing on of characteristics (traits) from parents to offspring

• Genetics is the study of heredity

KEY CONCEPT Mendel’s research showed that traits are inherited as discrete units.

Mendel laid the groundwork for genetics.

• Traits are distinguishing characteristics that are inherited.

• Genetics is the study of biological inheritance patterns and variation.

• Gregor Mendel showed that traits are inherited as discrete units.

• Many in Mendel’s day thought traits were blended.

Mendel’s experiment

• Go to film clip from biology text book

Mendel used peas...• They reproduce sexually• They have two distinct, male and female, sex cells called gametes

• Their traits are easy to isolate

Mendel crossed them

• Fertilization - the uniting of male and female gametes

• Cross - combining gametes from parents with different traits

Mendel’s data revealed patterns of inheritance.

• Mendel made three key decisions in his experiments.– use of purebred plants– control over breeding– observation of seven

“either-or” traits

• Mendel used pollen to fertilize selected pea plants.

Mendel controlled thefertilization of his pea plantsby removing the male parts,or stamens.

He then fertilized the femalepart, or pistil, with pollen froma different pea plant.

– P generation crossed to produce F1 generation

– interrupted the self-pollination process by removing male flower parts

• Mendel allowed the resulting plants to self-pollinate.

– Among the F1 generation, all plants had purple flowers

– F1 plants are all heterozygous

– Among the F2 generation, some plants had purple flowers and some had white

• Mendel observed patterns in the first and second generations of his crosses.

• Mendel drew three important conclusions.

– Traits are inherited as discrete units.– Organisms inherit two copies of each gene, one from each parent.– The two copies segregate

during gamete formation.– The last two conclusions are

called the law of segregation.

purple white

What Did Mendel Find?

• He discovered different laws and rules that explain factors affecting heredity.

Rule of Unit Factors

• Each organism has two alleles for each trait

• Alleles - different forms of the same gene

• Genes - located on chromosomes, they control how an organism develops

Rule of Dominance

• The trait that is observed in the offspring is the dominant trait (uppercase)

• The trait that disappears in the offspring is the recessive trait (lowercase)

Law of Segregation

• The two alleles for a trait must separate when gametes are formed

• A parent randomly passes only one allele for each trait to each offspring

Law of Independent Assortment

• The genes for different traits are inherited independently of each other.

Phenotype & Genotype

• Phenotype - the way an organism looks

• red hair or brown hair

• genotype - the gene combination of an organism

• AA or Aa or aa

Heterozygous & Homozygous

• Heterozygous - if the two alleles for a trait are different (Aa)

• Homozygous - if the two alleles for a trait are the same (AA or aa)

Dihybrid vs Monohybrid

• Dihybrid Cross - crossing parents who differ in two traits (AAEE with aaee)

• Monohybrid Cross - crossing parents who differ in only one trait (AA with aa)