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The Study of Inheritance

Gregor Mendel – The Father of Genetics

• Austrian monk

• Published ideas in 1866

• Worked with garden peas

• Determined basic genetics principles.

Garden Peas• Self pollinating so

Mendel didn’t have to worry about bees and wind, etc.

• Traits are contrasting

• Easily controlled

• Can be eaten at end

Traits studied by Mendel

round wrinkled

Crosses by Mendel:

round wrinkled

All round

¾ round ¼ wrinkled

Round Wrinkled seeds

Simple Steps for Solving Genetics Problems:

1. Determine what the question is asking 2. Write a key with information given:

Ex: KEY: TT = tongue rollerTt = tongue rollertt = nontongue roller

Use letters given or make up your own – be sure to include all possible genotypes and the phenotypes they produce.

Keep the key handy – I must be able to find it easily

3. Determine what the cross is: read carefully and figure out which two genotypes are being crossed: – Ex – if the problem states that a

heterozygous maN is married to a woman who can’t roll her tongue, the cross would be:

• Tt x tt

• If the problem states that the P1 generation is one homozygous dominant crossed with a homozygous recessive, the cross would be: – TT x tt

4. Draw a Punnett SquareWrite the cross _____ x _____

Ex: Tt x Tt

Determine which sex cells will be possible by each parent. The sex cells of the individual on the left go down the side of the square, those of the parent On the

right, go across the top of the square:

NOW YOU CAN FIGURE OUT THE POSSIBLE TYPES OF OFFSPRING THEY MAY HAVE

T t

T

t

Move each gamete over or down and then make pairs of gametes to show what genes will be expected to be in the offspring.

T t

T TT

Tt

t Tt tt

5. Now you can show the genotypic and phenotypic ratios

• First, do the genotypic ratio – GR– Count how many of the offspring will have

each genotype• Ex – GR = 1TT:2Tt:1tt

• Then do the phenotypic ratio – PR• Ex – PR = 3 tongue-rollers:1 nontongue roller

Possible Questions Regarding a Cross:

• Can they have a child who is a tongue roller?

• Can they have a heterozygous tongue-rolling child?

• Can they have a child who is a homozygous tongue roller?

Other types of problems• You might be given the offspring and asked to determine

the possible cross: – Ex – If two brown dogs had a litter of puppies that

included 3 brown and 1 white, what are the most likely genotypes of their parents?

1. Write a key2. Write the cross, leave genotypes blank3. Write the kinds of offspring4. Then figure out what the parents were

Key: brown is dominant so use B = brown and b = white

BB = brown ______ x _______

Bb = brown 3 brown 3 brown:1white ratio

bb = white 1 white 3 B_ : 1 bb

The bb got one b from the mother, the other from the father, so each parent must have a small b. Since both parents are brown, they must also have a B, making them both Bb

Incomplete dominance

• Also called

co-dominance.

• Neither allele is

dominant and when

hybrid, the phenotype

is mixed.

Solving Incomplete Dominance Problems

• Since neither gene is dominant, use two different capital letters in your key:

Ex: Key RR = redCross = RW x RW RW = pink WW = white

GR =PR =

R W

R

W

Sex-Linked traits : gene is found on X-chromosome, not the Y

• Genes for colorblindnessand hemophilia are common in humans

• Females have two genes for this trait

• Males have only one gene – so they have no chance to be a carrier

Are you colorblind?Can you see the shapes?

Sex-linked Traits – hemophilia pedigree in royal families of Europe

Current royalty in England

Russian family before

revolution

Sex-Linked Traits – the gene for that trait is on the

X-chromosome, not on the Y-chromosome

• Ex: in humans, both colorblindness and hemophilia are sex-linked

• Key for colorblindness: – XBXB = normal visioned female

XBXb = normal visioned femaleXbXb = colorblind femaleXBY = normal visioned maleXbY = colorblind male

Punnett Square for colorblindness:

Xb Y

XB

XB

Cross = XBXB x XbY

• Would you expect any of their children to be colorblind?

• What happens when the carrier daughters grow up and marry men who have normal vision? – Do that cross at your seats….Can

they have colorblind children? Colorblind daughters?

Colorblind sons?

Multiple Alleles• Some traits are controlled by more than two

alleles that can be inherited. Each person only inherits two of those alleles – one from each parent.

• This is the case with human blood types: – There are 3 alleles in the population:

• IA allele

• IB allele• i allele – recessive to the

other alleles

Incompletely dominant to each other

A Blood test will show which type you are:

ABO Blood type crosses:

• The key is complicated: – IAIA or IAi = type A– IBIB or IBi = type B– IAIB = type AB– ii = type O