Introduction to GeneticsChapter 11

What is genetics?• Genetics – the scientific study of heredity.– * Heredity is the passing of traits from parents to

offspring.

• A trait is a specific characteristic that varies from one individual to another.

The Work of Gregor Mendel• Gregor Mendel – the father of genetics.

– Born in Austria in 1822

• He was a priest, a teacher, and a gardener

– He made many observations of pea plants in his monastery garden which revolutionized the study of inheritance.

The Work of Gregor Mendel• Mendel’s Experiments

– Mendel had several different “true-breeding” lines of pea plants, each with different traits.

– Mendel cross-pollinated the different lines of pea plants and studied the inheritance of traits in the offspring.

The Work of Gregor Mendel• Mendel’s F1 Results

Seed

Shape

Seed

Color

Seed

Coat

Color

Pod

Shape

Pod

Color

Flower

PositionPlant

Height

P

Round Yellow Gray Smooth Green Axial Tall

Wrinkled Green White Wrinkled Yellow Terminal Short

F1

100%

Round

100%

Yellow

100%

Gray

100%

Smooth

100%

Green

100%

Axial

100%

Tall

The Work of Gregor Mendel

• Mendel’s Early Conclusions

1) Inheritance is determined by “factors”(genes) that are passed from parents to offspring.

- We now know that a gene is a segment of DNA that can be thousands to millions of base pairs long and codes for a trait.

2) Each of these “factors” (genes) has different versions (alleles)

3) The principle of dominance: Some alleles are dominant and block out others that are recessive.

The Work of Gregor Mendel

• Mendel’s Next Experiment

– Mendel wanted to find out if the recessive alleles still existed in the F1 plants.

– To determine this, he allowed the F1 plants to self-pollinate.

– He then studied the inheritance of the offspring (F2 plants)

The Work of Gregor Mendel• Results of the F1 Cross

– The recessive alleles reappeared!

• About 25% of the resulting F2 plants had the recessive trait.

The Work of Gregor Mendel• Explaining the F1 Cross

– Each F1 plant had one dominant and one recessive allele for the trait being studied.

– The principle of segregation: The alleles from each parent are separated during the formation of gametes (sperm and egg).

• In the F1 cross, ½ of the gametes received the dominant allele and the other ½ received the recessive allele.

– When the plants self-pollinated, two gametes came together (fertilization).

• If both gametes contained the recessive allele, the resulting plant showed the recessive trait. (25% chance)

• If one or both of the gametes contained the dominant allele, the resulting plant showed the dominant trait. (75% chance)

Punnett Squares• Punnett Square – a diagram used to

predict the outcomes of a genetic cross.

– The types of gametes produced by both parents go on the outside of the square.

– The possible gene combinations of the offspring appear in the four boxes that make up the square.

Punnett Squares• An organism is said to be homozygous

for a trait if both alleles for that trait are the same. (RR or rr)

• An organism is said to be heterozygousfor a trait if both alleles for that trait are different. (Rr)

Punnett Squares

• Phenotype – the physical characteristics of an organism. (Tall or short)

• Genotype – the genetic makeup of an organism. (TT or Tt or tt)

• Practice problem #1:

– In pea plants, the allele for tall plants (T) is

dominant to the allele for short plants (t).

Perform the following cross:

• A heterozygous tall plant X a short plant.

Genotype Ratio = 2/4 Tt : 2/4 tt

Phenotype Ratio = 2/4 tall plants : 2/4 short plants

T t

t

t

Tt

Tt

tt

tt

• Practice problem #2:

– In a breed of dog, the allele for long tails (L) is

dominant over the allele for short tails (l). If

you cross two heterozygous long-tailed dogs,

what is the chance of a puppy having a short

tail?

Genotype Ratio = 1/4 LL : 2/4 Ll : 1/4 ll

Phenotype Ratio = 3/4 long tails : 1/4 short tails

25% chance

L

L

l

l

LL

Ll

Ll

ll