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Sex-linked Inheritance �& �Color Vision Deficiency

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Objectives

  • 1. Differentiate subjects showing wild-type phenotype from those showing mutant phenotype on a pedigree chart.

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  • 2. Explain why males have a higher prevalence of X-linked traits such as color vision deficiency (Color blindness).

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  • 3. Explain why females need two mutant chromosomes to show an X-linked recessive disorder while males only need one.

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Who decides the sex of a child?

  • A. Mother

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  • B. Father

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  • C. Both Mother and Father

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  • D. Priest/Pastor

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Which is the BEST definition of a gene?�

a. A picture collage of all the chromosomes in an individual.

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b. Functional units of biological information.

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c. The simplest carrier of genetic information.

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d. It’s a trick question! None of them are correct.

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Key Words

  • Genotype - the genetic constitution of an individual organism
  • Phenotype - observable characteristics

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  • Recessive – Needs a mutant gene from each parent to show mutant phenotype
  • Dominant - Needs one mutant gene to show mutant phenotype

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  • “Wild-type” - a gene or characteristic that prevails in natural conditions
  • Mutant – The gene has been changed

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Punnett Square

Punnett Squares are used by biologists to determine the probability of an offspring having a particular genotype.

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This can be used to determine the chances of a particular phenotype.

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Dominant vs. Recessive

  • Dominant Genes only need one copy for their phenotype to show. In this Punnett square which is dominant, brown eyes or blue eyes?

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X-Linked Inheritance

  • Humans carry a pair of sex chromosomes.
    • 1 Chromosome comes from the mother (Always an X)
    • 1 Chromosome comes from the father (Either an X or Y)

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  • In recessive inheritance, if a gene on the X chromosome is mutated, males will always show the mutated phenotype. Females will only show the mutated phenotype if they receive a mutated X from each parent.

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Scenario 1

  • A man with red-green color vision deficiency and a woman showing normal genotype has a child together. What are the chances that their son will be red-green colorblind? What about a daughter?
  • Determine father and mother genotypes.

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  • Add the father's genotype to the left side of the Punnett square and the mother's genotype on top.

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  • Pair the chromosomes in each of the 4 boxes to predict the genotype of the offspring.

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Scenario 1

  • A man with red-green color vision deficiency and a woman showing normal genotype has a child together. What are the chances that their son will be red-green colorblind? What about a daughter?

XX

XX

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XY

XY

Father’s Genotype? XY

Mother’s Genotype? XX

X

Y

X

X

2 Carrier Females

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2 Unaffected Males

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Scenario 1

  • Y = p =0.7 P^2+2pq+q^2

(0.49)^2+2

.49+ 0.42 +0.09 =1

y= q = 0.3

Yy

p^2

yy

PQ

Yy

PQ

yy

q^2

genotype frequencies of YY=0.489

Yy=0.42

yy=0.09

y

y

Y

y

2 Carrier Females

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2 Unaffected Males

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Scenario 2

  • A man who DOES NOT have red-green color vision deficiency and a woman who does have color vision deficiency have a child. What are the chances that their son will have red-green color vision deficiency? What about a daughter?

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  • Determine father and mother genotypes.

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  • Add the father’s genotype to the left side of the Punnett square and the mother’s genotype on top.

​

  • Pair the chromosomes in each of the 4 boxes to predict the genotype of the offspring.

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Scenario 2

  • A man who DOES NOT have red-green color vision deficiency and a woman who does have color vision deficiency have a child. What are the chances that their son will have red-green color vision deficiency? What about a daughter?

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XX

XX

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XY

XY

X

Y

X

X

2 Female Carriers

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2 Affected Males

Father’s Genotype? XY

Mother’s Genotype? XX

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Scenario 3

  • A man with red-green color vision deficiency and a woman who is a carrier for red-green color deficiency have a child. What are the chances that the their son is colorblind? What about a daughter?
  • Determine father and mother genotypes.

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  • Add the father’s genotype to the left side of the Punnett square and the mother’s genotype on top.

​

  • Pair the chromosomes in each of the 4 boxes to predict the genotype of the offspring.

​

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Scenario 3

  • A man with red-green color vision deficiency and a woman who is a carrier for red-green color deficiency have a child. What are the chances that the their son is colorblind? What about a daughter?

XX

XX

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XY

XY

Father’s Genotype? XY

Mother’s Genotype? XX

X

Y

X

X

One Carrier Female

One Affected Female

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One Normal Male

One Affected Male

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Pedigree

  • A pedigree is a genetic representation of a family tree that diagrams the inheritance of a trait or disease through several generations

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Can an affected father pass the gene for colorblindness to his son?

  • A. Yes

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  • B. No

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  • C. Sometimes

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True or False:

  • A male can have one mutant X chromosome and still show wild-type phenotype for red-green colorblindness.

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  • A female has to have two mutant X chromosomes to be red-green colorblind.

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  • The only way a male can be color blind is if he inherited a mutant X from his mother.