Genetics

Reference: Biology Textbook,

Chapter 11: Introduction to Genetics

Gregor Mendel

In 1865, Gregor Mendel published studies of inheritance in pea plants.

During sexual reproduction, male and female gametes (reproductive cells; n) join to form a zygote (2n) in the process known as fertilization.

Heredity is…

1. Controlled by “factors.” (known as genes on chromosomes)

2. Passed from generation to generation.

3.Predicted Outcomes through Probability using Punnett Squares.

1902 – Chromosome Theory of Heredity

Genes (Mendels “factors”) are located on chromosomes.

The different forms (variations) of a gene are called Alleles.

This is the basis for the modern science of Genetics.

Genetic Principles Dominant and Recessive

Dominant = seen when present on the chromosome Recessive = only seen when dominant allele is not present on

the chromosome

Law of Segregation and Independent Assortment Randomness Placement of chromosomes within the process of meiosis

Genotype and Phenotype Genotype – using letters to represent the trait’s alleles

Ex: Flower Color - FF, Ff, ff Phenotype – using descriptive words (adjectives) to describe

the trait’s alleles. Ex: Flower Color – Purple or white

Homozygous and Heterozygous Homozygous = same allele variations (TT, tt) Heterozygous = difference allele variations (Tt)

Genes VS Alleles

Gene

Allele

Genes VS Alleles

Gene Plant Height Flower Color

AlleleTall Plant Purple Flowers

Short Plant White Flowers

R.C. Punnett

In 1905, R.C. Punnett introduced the Punnett square.

Punnett squares can be used to predict and compare the genetic variations that will result from a cross (pg. 268).

Using Punnett Squares

Yellow pea seed (Y) is DOMINANT to green (y).

Genotype: YY x yy

Phenotype: Yellowx green

Parents (P)

Y Y

y

y

Using Punnett Squares

Yellow pea seed (Y) is DOMINANT to green (y).

Genotype: YY x yy

Phenotype: Yellowx green

Parents (P)

Y Y

y

y

Using Punnett Squares

Yellow pea seed (Y) is DOMINANT to green (y).

Genotype: YY x yy

Phenotype: Yellowx green

Parents (P)

Y Y

y Yy

y

Using Punnett Squares

Yellow pea seed (Y) is DOMINANT to green (y).

Genotype: YY x yy

Phenotype: Yellowx green

Parents (P)

Y Y

y Yy

y

Using Punnett Squares

Yellow pea seed (Y) is DOMINANT to green (y).

Genotype: YY x yy

Phenotype: Yellowx green

Parents (P)

Y Y

y Yy

y Yy

Using Punnett Squares

Yellow pea seed (Y) is DOMINANT to green (y).

Genotype: YY x yy

Phenotype: Yellowx green

Parents (P)

Y Y

y Yy

y Yy

Using Punnett Squares

Yellow pea seed (Y) is DOMINANT to green (y).

Genotype: YY x yy

Phenotype: Yellowx green

Parents (P)

Y Y

y Yy Yy

y Yy

Using Punnett Squares

Yellow pea seed (Y) is DOMINANT to green (y).

Genotype: YY x yy

Phenotype: Yellowx green

Parents (P)

Y Y

y Yy Yy

y Yy

Using Punnett Squares

Yellow pea seed (Y) is DOMINANT to green (y).

Genotype: YY x yy

Phenotype: Yellowx green

Parents (P)

Y Y

y Yy Yy

y Yy Yy

Using Punnett Squares

Yellow pea seed (Y) is DOMINANT to green (y).

Genotype: YY x yy

Phenotype: Yellowx green

All (100%) offspring in F1 Generation are Yellow hybrids.

Parents (P)

Y Y

y Yy Yy

y Yy Yy

Monohybrid Cross

Yy x Yy

Yellow x Yellow

F1 Generatio

nY y

Monohybrid Cross

Genotype: Yy x Yy

Phenotype: Yellow x Yellow

F1 Generatio

nY y

Y

y

Monohybrid Cross

Genotype: Yy x Yy

Phenotype: Yellow x Yellow

F1 Generatio

nY y

Y

y

Monohybrid Cross

Genotype: Yy x Yy

Phenotype: Yellow x Yellow

F1 Generatio

nY y

Y YY

y

Monohybrid Cross

Genotype: Yy x Yy

Phenotype: Yellow x Yellow

F1 Generatio

nY y

Y YY

y

Monohybrid Cross

Genotype: Yy x Yy

Phenotype: Yellow x Yellow

F1 Generatio

nY y

Y YY

y Yy

Monohybrid Cross

Genotype: Yy x Yy

Phenotype: Yellow x Yellow

F1 Generatio

nY y

Y YY

y Yy

Monohybrid Cross

Genotype: Yy x Yy

Phenotype: Yellow x Yellow

F1 Generatio

nY y

Y YY Yy

y Yy

Monohybrid Cross

Genotype: Yy x Yy

Phenotype: Yellow x Yellow

F1 Generatio

nY y

Y YY Yy

y Yy

Monohybrid Cross

Genotype: Yy x Yy

Phenotype: Yellow x Yellow

F1 Generatio

nY y

Y YY Yy

y Yy yy

Monohybrid CrossGenotype: Yy x Yy

Phenotype: Yellow x Yellow

Phenotypic Ratio:

75% (3/4) offspring are Yellow; Ratio is 3:4 (Yellow : total)

25% (1/4) offspring are green; Ratio is 1:4 (green : total)

F1 Generatio

nY y

Y YY Yy

y Yy yyGenotype Ratio:

25% (1/4) offspring are YY, Ratio is 1:4 (homozygous dominant : total)

50% (2/4) offspring are Yy, Ratio is 1:2 (heterozygous : total)

25% (1/4) offspring are yy, Ratio is 1:4 (homozygous recessive : total)

Dihybrid Cross

Round pea seed (R) is dominant to wrinkled pea seed (r).

FOIL: YyRr YR, Yr, yR, yr

(These are the possible genotypic combinations)

Dihybrid CrossRound pea seed (R) is dominant to

wrinkled pea seed (r). Possible

Combinations

YR Yr yR yr

YR

Yr

yR

yr

Dihybrid CrossRound pea seed (R) is dominant to

wrinkled pea seed (r). Possible

Combinations

YR Yr yR yr

YR YYRR YYRr YyRR YyRr

Yr YYRr YYrr YyRr Yyrr

yR YyRR YyRr yyRR yyRr

yr YyRr Yyrr yyRr yyrr

What is the probability that the plant will produce:

Yellow, Round Peas:

Yellow, Wrinkled Peas:

Green, Round Peas:

Green, Wrinkled Peas:

Possible

CombosYR Yr yR yr

YRYYRR

Yellow, Round

YYRrYellow, Round

YyRRYellow, Round

YyRrYellow, Round

YrYYRr

Yellow, Round

YyrrYellow,

wrinkled

YyRrYellow, Round

YyrrYellow,

wrinkled

yRYyRR

Yellow, Round

YyRrYellow, Round

yyRRgreen, Round

yyRrgreen, Round

yrYyRr

Yellow, Round

YyrrYellow,

wrinkled

yyRrgreen, Round

Yyrrgreen,

wrinkled

What is the probability that the plant will produce:

Yellow, Round Peas: 9/16

Yellow, Wrinkled Peas: 3/16

Green, Round Peas: 3/16

Green, Wrinkled Peas: 1/16

Possible

CombosYR Yr yR yr

YRYYRR

Yellow, Round

YYRrYellow, Round

YyRRYellow, Round

YyRrYellow, Round

YrYYRr

Yellow, Round

YyrrYellow,

wrinkled

YyRrYellow, Round

YyrrYellow,

wrinkled

yRYyRR

Yellow, Round

YyRrYellow, Round

yyRRgreen, Round

yyRrgreen, Round

yrYyRr

Yellow, Round

YyrrYellow,

wrinkled

yyRrgreen, Round

Yyrrgreen,

wrinkled

To find the probability that two different events will happen at the same time, multiply the

probabilities of each event happening

separately.

To find the probability that two different events will happen at the same time, multiply the

probabilities of each event happening separately.

Probability of offspring having YELLOW pea seeds:

¾

Probability of offspring having ROUND pea seeds:

¾

Probability of offspring having BOTH YELLOW AND ROUND PEAS:

¾ X ¾ = 9/16

Trihybrid Cross

Tall Stem (T) is dominant to short stem (t).

The probability of producing tall stems in a monohybrid cross is:

What is the probability that the offspring in a trihybrid cross will be Yellow, Round, and Tall?

Trihybrid Cross

Tall Stem (T) is dominant to short stem (t).

The probability of producing tall stems in a monohybrid cross is:

¾ What is the probability that the offspring in

a trihybrid cross will be Yellow, Round, and Tall?

¾ X ¾ X ¾ = 27/64