mendel genetics and punnett squares -...

Mendel Genetics and Punnett Squares

Origin of Genetics

Steps of a Monohybrid cross

Terminology

Practice

Punnett Squares

Determining Unknown Genotypes

The Origin of Genetics

• Heredity- the passing of characteristics from parents to offspring

• Genetics- the branch of Biology that focuses on heredity

• Cross- combining traits from two parents by breeding (sexual reproduction)


• Gregor Mendel – Austrian monk

– Studied theology, science and mathematics

– Noticed patterns in the pea plants he was growing

– Mendel crossed pea plants and studied the genetic outcomes

– Was the first to develop rules that accurately predict patterns of heredity

– Father of genetics


Monohybrid cross- a cross that involved one pair of contrasting traits

Ex. Crossing a white flower with a purple flower.

Steps of a Monohybrid cross

1. Ensure each variety is true-breeding for a particular characteristic

Ex. A true-breeding purple flower will produce only purple flowers

• These are the 1st two individuals crossed

- P generation (parental generation)

Steps of a Monohybrid Cross

2. Cross-pollinate two P-generation plants

Ex. A true breeding purple flower crossed with a true-breeding white flower

• The offspring are called F1 generation (filial generation)

Steps of a Monohybrid Cross

3. Self-pollinate the F1 generation to create the F2

generation (second filial generation)

– This creates a 3:1 ratio

True breeding P generation F1 generation F2 generation

Self pollinate

Self pollinate

Cross

Self pollinate

Terminology

• Alleles- alternative versions of genes

– The gene for hair color can have multiple alleles (blonde hair allele, brown hair allele, etc)

• Dominant- the expressed (visible) trait

• Recessive- the trait that is not expressed or observable

Terminology

• Homozygous- when two alleles of a particular gene are the same

– (PP), (pp), (RR), (rr), (VV), (vv), etc

• Heterozygous- when two alleles of a particular gene are different

– Only the dominant allele is expressed while the recessive is present, but not expressed

– (Pp), (Rr), (Vv), etc

Terminology

• Genotype- the set of alleles an individual has for a trait

– think “genes” and “letters”

– (Rr), (RR) or (rr)

– (Vv), (VV) or (vv)

• Phenotype- the physical appearance of a trait

– Think “face” and “colors”

– White, purple, red, blonde, freckles, blue eyes, etc

Practice

(assume that P is purple and p is white)

(Pp):

What is the genotype?

What is the phenotype?

Is it homozygous or heterozygous?

Practice


(pp):




Practice


(PP):




Punnett Squares

• Punnett square- is a diagram that predicts the outcome of a genetic cross by considering all possible combinations

Punnet Squares • Ex. Having freckles (F) is dominant to not

having freckles (f). Parent 1 is heterozygous for freckles and Parent 2 is also heterozygous

Parent 1

Parent 2

F

F F F F

F f

f

f

f f f

Phenotype ratio?

3:1

Punnett Squares

Box 1 is homozygous for….

Box 2 and 3 is…..

Box 4 is….

F

F F F

F

F

f

f

f

f f f

1 2

3 4

Punnett Squares

• Ex. A homozygous yellow pea plant (Y) is dominant over a homozygous green pea plant (y).

Y Y

y

y

Y

Y

Y

Y

y y

y y

What’s the genotype of all offspring?

What’s the phenotype of all offspring?

Yy

yellow What’s the phenotypic ratio?

4:0


• When a phenotype is dominant but the genotype is not known, a test cross is performed.

– Cross the unknown genotype with a homozygous recessive individual


What does that mean?

You can physically see that the pea plant is yellow, but you don’t know if they are (YY) or (Yy).

So, you cross the yellow pea plant with a recessive pea plant (yy) that is green.

If the offspring are all yellow the genotype is (YY)

If the offspring are ½ yellow and ½ green the genotype is (Yy)


• Use the Punnett Square and work backwards!

Known, recessive genotype

Unknown

genotype

y y

Y

???

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