Do You Know These WordsDo You Know These Words• Heredity

– Passing on of characteristics from parent to offspring• Genetics

– Part of biology that studies heredity• Gene

– Piece of DNA that codes for a polypeptide (specify certain trait)• Trait

– Characteristic that is inherited• Homologous Chromosomes

– Chromosome of the same type (pairs)• Locus

– Specific location of a gene on a chromosome• Diploid

– 2 copies of each chromosome = 2n• Haploid

– 1 copy of each chromosome = 1n

Gregor Mendel

• Austrian monk• A mathematician

aware of probability• The “father” of

genetics• Worked with pea

plants

Why Peas?

• Peas have many traits with 2 different forms that are easy to distinguish (ex: green or yellow seeds; tall or short plants)

• Could easily control mating of the pea plants through cross-pollination

• Pea plants are small, they grow quickly and easily, and they produce a lot of offspring.

Mendel’s Crosses

• In one cross, Mendel crossed tall pea plants with short pea plants

• All of the offspring were tall!• He then crossed two of the offspring tall plants• The next offspring were ¾ tall and ¼ short• Mendel said that 2 factors control each trait

(these are genes found on chromosomes)• Each form of a gene is called an allele.• The two alleles are found on homologous

chromosomes.

Mendel’s Crosses

Parent x Parent = P1 generation

Offspring x Offspring = F1 generation

Offspring = F2 generation

Mendel’s Law of Dominance

• Mendel called the observable trait (all tall plants) dominant and the trait that disappeared (short plants) recessive

• The allele for tall plants is dominant

• The allele for short plants is recessive

• If T = tall and t = short– TT = tall plant, Tt = tall plant, tt = short plant

• Each parent passes on (at random) to offspring only one allele for each trait!

• A plant that is Tt can pass either T or t in a gamete.

• Ex: Humans have ____ chromosomes.– When eggs and sperm are made, only half of these

chromosomes are passed…one of each kind from mom and one of each kind from dad

– That way a fertilized egg (and the baby) will have the normal # of chromosomes

• Diploid cells have pairs of genes on pairs of homologous chromosomes.

• The 2 genes of each pair are separated during meiosis and end up in different gametes.

More vocabulary!

• Phenotype = individual’s observable traits– Ex: (tall, short)

• Genotype = genes an individual carries– Ex: TT, Tt

• Homozygous = has 2 of the same alleles– Purebred, true-breeding– TT, tt

• Heterozygous = has 2 different alleles– Hybrid– Tt

Monohybrid Cross

• Cross a single trait• Cross 2 parents that

are homozygous for different alleles of a single trait

• Saw purple allele is dominant to (masks) white allele

Purple flower x White flower

All purple flower plants

705 purple224 white

About a 3:1 ratio

Now lets investigate the heredity seen in Reebops!

Punnett Squares

• Can be used to show all possible offspring!

• Ex.: R = round seed, r = wrinkled seed• Homozygous round x homozygous wrinkled• What F1 offspring can be produced? What are

the genotypes and phenotypes?

F2 Generation

• Now let’s cross two of the heterozygous offspring from the F1 generation.

Dihybrid Cross

• Cross two traits

• Start with 2 parents that are homozygous for different alleles for 2 traits

• Round Yellow Seed x Wrinkled Green Seed

R= round Y = yellow

r = wrinkled y = green

Punnett Square• Example: RRYY x rryy

Mendel’s Theory of Independent Assortment

• Gene pairs for different traits are inherited independently of each other

• Ex: Genes for round seed shape are inherited independently of genes for yellow seed color!

• Note: We will see later that this does not always apply

Test Cross

• I have a guinea pig that is black.

• I know that the black allele (B) is dominant to the brown allele (b).

• What are the possible genotypes for my guinea pig?

• I want to breed guinea pigs, and I want to have pure-bred guinea pigs!

• How do I know if he is pure-bred?

Test Cross

• My guinea pig may be BB or Bb. Both would show the black phenotype!

• To see if a dominant phenotype is due to a homozygous dominant or a heterozygous phenotype, I can…– Cross it with one that is homozygous

recessive! B B B bb bb b