MENDELIAN GENETICS

BiologyCorsicana High

School

Genetics

•the branch of biology that studies how traits are inherited from generation to generation

Heredity vs Environment

•heredity---the passing on of traits from parent to offspring

•environment----outside forces that act on an organism

•both heredity and environment affect traits

Types of Traits•species traits---are the

same for all members of a species

•individual traits---make individuals different from others in the same species

“Recent” Discoveries

•(within the last 100 years)•Genetic information is carried on

chromosomes•Chromosomes are made of DNA•However, 100 years before Watson &

Crick, we knew the “laws” of heredity---how traits are passed on

Gregor Mendel•“Father of Heredity”•an Austrian monk who lived in the

middle 1800s; high school teacher•first person to discover the mechanism

for the inheritance of traits•saw regular patterns in flowers and

other plants he raised

Mendel’s Experiments

•He chose the garden pea for his studies because:–34 different varieties–normally self-fertilizing–easily cross-fertilized by hand–could be prevented from accidental self-fertilization

Mendel’s Experiments (continued)

•He conducted a 7-year series of experiments on the peas

•His results were virtually the same for all traits he studied

Mendel’s Results•Example: round x wrinkled peas

–F1 generation: all peas were round

•he let this generation self-fertilize–F2 generation: 3 round :

1 wrinkled

Generations•F1 = first filial generation---

the first offspring of two “pure-breeding” plants

•F2 = second filial generation-the second generation, achieved by letting the F1 self-fertilize

Analysis of his Results

•Since he got virtually the same results on all traits studied, Mendel theorized that there must be laws to govern inheritance of traits

•He based his explanations on the laws of probability

Mendel’s Hypotheses

•Concept of Unit Characters---inherited traits are controlled by factors (genes) that occur in pairs

•Principle of Dominance and Recessiveness---one gene in a pair may mask the other, preventing it from having effect

Mendel’s Hypotheses

•Law of Segregation---a pair of genes is separated in forming gametes

•These three laws, taken together, explain Mendel’s results

Mendel’s Hypotheses

•Law of Independent Assortment---when the gene pairs (on a given pair of chromosomes) are separated, they are distributed to the gametes completely independently of the way other gene pairs (on other chromosomes) are distributed

Mendel’s Results

round x wrinkled

P1

(parent)

RR rr

sperms

eggs

zygote

F1

R R r r

Rr all round

Mendel’s Results (continued)

F1round x round (self-

fertilize)Rr RrR rsperm

seggs

Rr zygotes

3 round : 1 wrinkled

RR Rr rr

F2

rR

Genetics Vocabulary

•gene---a factor by which a trait is inherited

•allele---the alternate forms of a gene for a certain trait–example: round, wrinkled

Genetics Vocabulary (continued)

•dominant---a gene which will mask or cover up its allele –capital letter

•recessive---a gene which will be masked or covered up by its allele–lower case letter

Genetics Vocabulary (continued)•homozygous---the two genes for a

trait in an individual are exactly the same–example: RR, rr

•heterozygous---the two genes for a trait in an individual are different–example: Rr

Genetics Vocabulary (continued)•genotype---the genetic makeup of

an individual (the actual genes it has)---examples: RR, Rr, rr

•phenotype---the appearance of an organism as a result of its genotype (what it looks like)---examples: round, wrinkled, etc.

Monohybrid Cross

•a genetic cross dealing with one set of traits

•Punnett square---a grid for working genetic crosses

Example:

heterozygous round x heterozygous round

Rr x Rr

R

r

r

R R

R

R r

r r r

R

Genotypes

1 RR : 2 Rr : 1 rr

Phenotypes

3 round : 1 wrinkled

Codominance•traits in which neither allele

of a pair is dominant•the two genes blend

together to give an intermediate trait

Incomplete Dominance

• One allele is not completely dominant over another

• In incomplete dominance, the heterozygous phenotype is somewhere in between the two homozygous phenotypes

• Example: A red flowered parent (RR) is crossed with a white flowered parent(WW) and the offspring (RW) are pink

Multiple Alleles

• Genes that have more than two alleles

• It does not mean that one individual has more than two alleles, but that more than two exist in a population

• Example: Coat color in rabbits and human blood type

Polygenic Traits

• Traits that are controlled by two or more genes

• “Polygenic” means having many genes

• Skin color is controlled by at least four different genes. That is how we have such a wide variety of colors.

Dihybrid Cross

•a genetic cross dealing with two traits at a time

•predicts what combinations of traits will be found in the offspring

•Mendel’s Law of Independent Assortment applies to these crosses

Key to Working Dihybrid Crosses

• Each gamete gets one gene from each pair