Sci 30 Chapter 2.2 Punnett

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Punnett Squares

Science 30

Chapter 2.2 - page 96

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1. Punnett Square Definition:

• A table that uses the

alleles

of the

parents to show all the

possible

_possible

outcomes

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2. Rules of probability are relevant

to genetics.

• Express probability as a number

between 0-1, (decimal), as a fraction or

a percentage

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Example Problem: Punnett Square

• Two pea plants in the parent generation each

contain the dominant allele P that codes for

purple flowers and the recessive allele p that

codes for white flowers. Use a Punnett

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Example Problem: Punnett Square

• Step 1 … 2 … 3 … _{Fraction of offspring}

with genotype:

PP

p p

P p

Pp

PP: 2Pp : pp

25% :50% : 25%

PHENOTYPE

purple: white

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Example - Tongue rolling

• In human beings the ability to roll one’s

tongue is dominant over non-tongue rolling. • Identify Alleles =

• Can YOU roll? a) YES

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Example Problem: Punnett Square

• Step 1 … 2 … 3 … Fraction of offspring

with genotype:

R

_r

r

R r

r r

R r

r r

R r : r r

50% : 50%

PHENOTYPE

ROLL : NON-ROLL

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3. Identifying Alleles:

• _Heterozygous

_:

_two

_{different alleles,}

one

dominant

one

recessive

– _{Heterozygous for flower color =}

_{P p}

• This is the GENOTYPE (description of alleles using letters) =

P = purple OR p = white

– _{What color is this plant?}

_{P p= purple}

• This is the PHENOTYPE (what we

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Identifying Alleles

• Homozygous:

two

copies of the same

allele,

either

both dominant or both

recessive

–

_{Homozygous recessive for flower}

color = p p

• Phenotype of this plant is:

white

–

_{Homozygous dominant for flower}

color =

P P

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Practice

• For the following questions use the trait Tongue Rolling. Identify the alleles below:

Tongue Rolling =

R

Non-rolling =

r

1.Use letters to describe the genotype of each following individual:

a. A homozygous tongue roller

b. A heterozygous tongue roller

c. A homozygous non-roller

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Practice

2. The fuzzy skin of a peach is produced by a

dominant allele N, and the smooth skin of a

nectarine is produced by the recessive allele

n. State whether the skin phenotype of the

following individuals is fuzzy or smooth

a. NN

b. Nn

c. nn

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Practice

3. State the

most likely

genotype of

each example

a. A smooth skinned nectarine

b. A fuzzy-skinned peach bred from a cross between a peach – producing tree and a nectarine-producing tree

c. A fuzzy-skinned peach produced from a long line of peach-producing trees

Nn

NN

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Practice

4) Predict the following:

a)A smooth skinned nectarine tree is crossed with another smooth skinned nectarine … the offspring are then crossed with a

heterozygous fuzzy peach tree … what will be the genotypes and phenotypes of the resulting offspring?

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4. Monohybrid Problems

• One trait being followed from one generation to the next.

• _{Eg. A cross between a heterozygous}

peach and a homozygous nectarine

• Steps in doing Monohybrid Crosses:

1. Identify the gametes

2. Identify the parent cross 3. Draw the Punnett Square

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Example #1:

Homozygous tall X Homozygous pea plant recessive

1.Determine the genotypic and phenotypic ratios of the offspring

T t

T

t

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Example #2

• Heterozygous Tall X Heterozygous Tall

• Determine the genotypic and phenotypic ratios of the offspring

T T

t t

T t

T

t

T

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Do we get it?

1) Two heterozygous parents have a child – what is the percentage probability that their offspring will be able to roll their tongue?

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Do we REALLY get it?

2. A heterozygous man marries a

homozygous recessive woman.

What is the probability that their

offspring can roll their tongue?

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OTHER PATTERNS OF

INHERITANCE

Science 30

Unit 1 – Biology

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Other Pattern of Inheritance

(page 99)

• Multiple alleles_{Multiple alleles}:: Not all traits are controlled by one gene or only have two alleles for a gene.

– For example: hair and eye color are determined by many pairs of alleles

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Other Pattern of Inheritance

(page 99)

• _Codominance_Codominance_{in alleles: some times} both alleles are expressed equally.

– For example coat color in cattle. A

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Other Patterns of Inheritance

• Blood typing in humans reflects both of these patterns of inheritance:

– Three alleles: A, B and O produce four

blood types or phenotypes

– A and B alleles are

codominant and are both dominant over

O

Table of Blood Types Genotype Phenotype

AA or AO

A

BB or BO

B

AB

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HETEROCHROMIA IRIDIUM

• Eye color is a manifestation of the pigment that is present in the iris.

• Brown eyes are rich in melanin deposits, and blue eyes indicate a lack of melanin. • The melanocytes of the iris rest in a richly innervated psuedosyncytium, which is

necessary to maintain eye color.

• Two genes control eye color: EYCL3, found on chromosome 15, which codes for brown/blue eye color (BEY), and EYCL1, found on chromosome 19, which codes for green/blue eye color (GEY).

• Although previously believed to be inherited in simple Mendelian fashion, eye color has proved to be a polygenic trait.

• Precisely how these genes interact to provide the full constellation of colors, such as hazel and gray, is as yet unknown.

• Furthermore, other genes may determine the pattern and placement of pigment in the iris, thereby accounting for solid brown as opposed to rays of color.

• Heterochromia iridium (two different-colored eyes within a single individual) and

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Sex Chromosomes

• The X and Y chromosomes that determine gender also carry other genes that do not

determine the sexual characteristics

• Autosomal inheritance:

–

_{trait linked to}

autosomal

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Sex Chromosomes

• Sex-linked inheritance:

–

_{trait linked to sex chromosomes X or Y}

– More than 120 known sex-linked traits in humans

– _{Almost always found on the X chromosome,}

single recessive gene on the X will cause the disease

– Genes on Y only inherited by males

– Examples include:

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Sex-linked traits

• Red-Green color blindness is the inability to

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Sex-Linked problems

• When solving sex-linked cross problems

the alleles used are the X and Y and the

trait is denoted with a superscript letter.

• If

N

is full color vision and

n

is

color-blind then:

– _{a man that is color-blind has the}

genotype

X

n

Y

_{and a woman who is a}

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Sex-linked problems

• _{Example One: What is the outcome of a}

cross between a normal male and a

woman who is a carrier for hemophilia?

X

N

Y

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Sex-linked problems

• Example Two: What is the outcome of a cross between a color blind man and a woman who is homozygous for full color vision?