Genes and Inheritance

A story of a Monk and his pea plants

Who was Gregor Mendel?● Father of Modern Genetics● Austrian monk born in 1822● Mendel studied inherited traits in pea plants

● Short or tall● Seed color (green or yellow)● Seed coat color (white or gray)● Pod shape (smooth or constricted)● Pod color (green or yellow)

Gregor Mendel and heredity

● Heredity is the passing of traits from one generation to the next.

● The study of heredity is called genetics.

● Mendel discovered genes.● Scientists use the word

gene to describe factors that control a trait.

● Mendel discovered that there are two traits for every gene.Mendel

Alleles

● The different forms of a gene are called alleles.● For example, the gene that controls stem height

in pea plants has two alleles. ● Each pea plant inherits one allele from each of its

parents.● The alleles for pea plant height are tall and short

stems.● An organisms traits are controlled by the alleles it

inherits from its parents.● Some alleles are Dominant, while other

alleles are recessive.

Dominant vs. Recessive Alleles

● A dominant allele is one whose trait always shows up in an organism when present.

● A recessive allele is a trait that is “hidden” when a dominant allele is present.

● There are times when two traits show up anyway. This is called codominance.

Consider the stem height in pea plants. The dominant allele is tall stems, represented by a capital T. The recessive allele is short stems, represented by a lower case t.

Dominant alleles are always represented by a capital letter. Recessive alleles are always represented by a lower case letter.

Tall Stem = T Short Stem = tDominant Recessive

When Mendel crossed purebred tall plants with short plants, he grew a generation of tall plants. Why? Let’s look at rules of inheritance in more detail.

Predicting Inheritance

To determine the chances of inheriting a given trait, scientists use Punnett squares and different symbols to represent the genes.

A Punnett square can be used to predict the probability that a trait will occur in an organism.

How to make a Punnett Square

1.) Start by drawing a box and dividing it into four squares.

2.) Write the male parents alleles along the top of the square and the female parent’s alleles along the left side.

3.) Copy the female parent’s alleles into the boxes to the right.

4.) Copy the male parent’s alleles into the boxes beneath them.

R r

R

r

R R

r r

R

R

r

r

Male parent’s alleles

Fem

ale

pare

nt’s

alle

les

R = round seeds

r = wrinkled seeds

How to make a Punnett Square

R r

R

r

R R

r r

R

R

r

r

The completed Punnett Square shows the possible allele combinations of the offspring.

Since “R” is the dominant allele, 75% (3 out of 4) of the offspring will have round seeds, while 25% (1 out of 4) will have wrinkled seeds.

R = round seeds

r = wrinkled seeds

Mendels’ Principle of Dominance

Some genes (alleles) are dominant and others are recessive. The phenotype (trait) of a dominant gene will be seen when it is paired with a recessive gene.

Tongue-rolling is a dominant trait. Brown eyes are dominant

over blue eyes.

Homozygous = RR, rr Heterozygous = Rr

Combination of two of the same alleles one inherited from each parent.

Combination of two different alleles one inherited from each parent.

SAME DIFFERENT

Mendels’ “Law” of Segregation

Each gene (allele) separates from the other so that the offspring get only one gene from each parent for a given trait.

Let’s consider the cross between a homozygous tall male pea plant and a homozygous short female pea plant. How do we determine the probability of inherited height traits in their offspring?

Punnett Squares

Homozygous tall male pea plant

Hom

ozyg

ous

shor

t fem

ale

pea

plan

t

t

t

T T

Tt Tt

Tt Tt

Inheritance and probability

● In other words, if we cross a purebred tall male pea plant (TT) with a purebred short female pea plant (tt), the F1 generation will produce 4 tall pea plants (Tt).

● The probability that an offspring will be tall (Tt) is 4 in 4 or 100%.

● The genotype of the offspring is Tt.● A genotype is the combination of alleles in the

offspring.● The phenotype of the offspring is a tall stem.

● The phenotype is the inherited trait of the offspring.● Let’s try a few more examples.

In humans, brown eyes (B) are dominant over blue (b). A heterozygous brown-eyed man marries a homozygous blue-eyed woman. Draw the Punnett square.

B b

b

b

Bb

Bb

bb

bb

Genotypes: Bb, bb

Phenotypes:

50% brown-eyed 50% blue-eyed

Can you curl your tongue up on the sides? Tongue-curling in humans is a dominant genetic trait. Suppose a man who is Tt for tongue-curling marries a woman who is also Tt for this trait. What arethe possible genotypes and phenotypes of their children and the percent chance for each?

T t

T

t

TT Tt

tT tt

Genotypes: TT, Tt, tt

Phenotypes: 75% tongue-curling, 25% can’t curl