Patterns of Inheritance (Mendelian Genetics). Gregor Mendel 1890’s Central European Monk Conducted research on pea plants Used garden peas Easy to grow.

Patterns of Inheritance(Mendelian Genetics)

Gregor Mendel 1890’s Central European Monk Conducted research on pea plants

Used garden peasEasy to growCan produce many generations quicklyCross-pollination

Mendel’s experimental design

Statistical analyses: Worked with large numbers of plants counted all offspring made predictions and tested them

Excellent experimentalist controlled growth conditions focused on traits that were easy to score chose to track only those characters that varied in an

“either-or” manner

GENE:

the unit of heredity

a section of DNA sequence encoding a single protein

Genetic Vocabulary Character: a heritable feature, such as

flower color Trait: a variant of a character, such as

purple or white flowers Each trait carries two copies of a unit of

inheritance, one inherited from the mother and the other from the father

Alternative forms of traits are called alleles

ALLELE:

Forms of genes

Ex: Purple flowers vs. white flowers

Mendel’s experimental design

Mendel also made sure that he started his experiments with varieties that were “true-breeding”

X

X

X X

X

X

Genetic Vocabulary Generations:

P = parental generation F1 = 1st filial generation, progeny of the P generation F2 = 2nd filial generation, progeny of the F1 generation

(F3 and so on) Crosses:

Monohybrid cross = cross of two different true-breeding strains (homozygotes) that differ in a single trait.

Dihybrid cross = cross of two different true-breeding strains (homozygotes) that differ in two traits.

GENOTYPE the specific

allelic combination for a certain gene or set of genes (PP, Pp, pp)

PHENOTYPE the physical

appearance of a particular trait (Purple, White)

Phenotype vs Genotype

Figure 14.6

3

1 1

2

1

Phenotype

Purple

Purple

Purple

White

Genotype

PP(homozygous)

Pp(heterozygous)

Pp(heterozygous)

pp(homozygous)

Ratio 3:1 Ratio 1:2:1

Mendel’s Experimental Design

Mendel mated 2 contrasting, true-breeding varieties, a process called hybridization

True-breeding parents are called the P generation

The hybrid offspring of the P generation are called the F1 generation

When F1 individuals self-pollinate the F2 generation is produced

Mendel’s Observations When Mendel crossed true-breeding white and purple flowered pea plants all of

the offspring were purple When Mendel crossed the F1 plants, many of the plants had purple flowers, but

some had white flowers A ratio of about three to one, purple to white flowers, in the F2 generation

EXPERIMENT True-breeding purple-flowered pea plants and white-flowered pea plants were crossed (symbolized by ). The resulting F1 hybrids were allowed to self-pollinate or were cross- pollinated with other F1 hybrids. Flower color was then observed in the F2 generation.

RESULTS Both purple-flowered plants and white-flowered plants appeared in the F2 generation. In Mendel’s experiment, 705 plants had purple flowers, and 224 had white flowers, a ratio of about 3 purple : 1 white.

Law of Segregation States that 2 alleles coding for the same

trait separate during gamete formation

HOMOZYGOUS: Having the same alleles for a gene

(PP, pp)

HETEROZYGOUS: having different alleles for a gene (Pp)

DOMINANCE An organism with a

dominant allele for a particular form of a trait will ALWAYS have that form

(P = purple)

RECESSIVE An organism with a

recessive allele for a particular trait will have that form only when the dominant allele is not present

Practice• use 2 letters to represent genotype• A capital letter represents the dominant form of a

gene (allele) and a lowercase letter represents the recessive form of the gene (allele).

• Example below: P=dominant purple and p= recessive white

The phenotype for this flower is violet while its genotype (if homozygous) is PP.

The phenotype for this flower is white while its genotype is pp (must have 2 of the recessive copies of the allele).

Punnett Squares standard way of

working out what the possible offspring of 2 parents will be helpful tool to show

allelic combinations and predict offspring ratios

Before we go further lets review how to set up a Punnett Square…

We begin by constructing a grid of two perpendicular lines.

Next, put the genotype of one parent across

the top and the other along the left side.

For this example lets consider a genotype of BB crossed with bb.

B B

b

b

• Notice only one letter goes above each box

• It does not matter which parent’s genotype goes on either side.

Fill in the boxes by copying the column & row

letters down and across into the empty spaces

B B

b B

B

B

Bb

b

b

b

b

Lets say:

W- dominant white

w- recessive violet

W wW

Parents in this cross are heterozygous (Ww).

Note: Make sure I can tell your capital letters from lowercase letters.

What percentage of the offspring will have violet flowers?

ANSWER: 25% (homozygous recessive)

Usually write the capital letter first

w

W W W w

W w w w

Red hair (R) is dominant over blond hair (r). Make a cross between a heterozygous red head and a blond.

RrRr rr rr

RrRr rr rr

R r

r

r

What percentage of the offspring will have red hair? 50%

Let’s try some more…

In pea plants, tall pea plants (T) are dominant

over short pea plants (t). Construct a Punnett

Square for a heterozygous tall pea plant and a short pea plant.

Tt tt

Tt tt

T t

t

t

What are the percentage of phenotypes?

50% tall

50% short

Black eyes (R) is dominant over red eyes (r)

in rats. Make a cross between a homozygous rat with black eyes and a rat with red eyes.

Rr Rr

Rr Rr

R R

r

r

What is the possibility of a red eye off springs?

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