Color Blindness & Baldness In People

Color Blindness In Humans: An X-Linked Trait

Pattern Baldness In Humans: Sex Influenced

1. Color Blindness In Humans: An X-Linked Trait

Note: This Is Sometimes Called A Sex-linked Trait

Test Yourself With The Table Below

Numbers That You Should See If You Are In One Of The Following
Four Categories: [Some Letter Choices Show No Visible Numbers]

	4 Sex-Linked Traits:
	1. Normal Color Vision: A: 29, B: 45, C: --, D: 26
	2. Red-Green Color-Blind: A: 70, B: --, C: 5, D: --
	3. Red Color-blind: A: 70, B: --, C: 5, D: 6
	4. Green Color-Blind: A: 70, B: --, C: 5, D: 2

See Chart of 216 Colors In HTML Code

Note: These Colors May Appear Slightly Different If You Are Red-Green Color Blind

Inheritance Of Color Blindness In Men & Women

Sex-Linked Genes Located On X Chromosome:

+ = Normal Vision (Dominant)
o = Color Blindness (Recessive)

Sex	Color-blind	Normal Vision
Male	X^oY	X⁺Y
Female	X^oX^o	X⁺X⁺ X⁺X^o

Cross Between A Color-blind Man (X^oY) and
Heterozygous Normal Vision Woman (X⁺X^o)

Gametes	X^o	Y^o
X⁺	X⁺X^o	X⁺Y
X^o	X^oX^o	X^oY

In the above cross, four different possible offspring are produced:

X^oY: Color-blind Boy (1/4 or 25%)

X⁺Y Normal Vision Boy (1/4 or 25%)

X^oX^o Color-blind Girl (1/4 or 25%)

X⁺X^o Heterozygous Normal Vision Girl (1/4 or 25%)

Note: The heterozygous normal vision girl carries the recessive gene for
color blindness. On the average, she will pass this gene on to half of her
sons and half of her daughters.

Frequency Of Normal & Color-blind Genes In A Population

The men's sperm carry one of the following three combinations: An X chromosome with the gene for normal vision (X⁺), an X chromosome with the gene for color-blindness (X^o), or a Y chromosome (Y). The women's eggs carry one of the following two combinations: an X chromosome with the gene for normal vision (X⁺), or an X chromosome with the gene for color blindness (X^o). She does not carry the Y chromosome (Y) in her eggs.

Gametes of
the Men &
Women in
Population

Decimal
Value
of the
Gametes

Sperm

X⁺
0.450

X^o
0.050

  Y^o
0.500

Eggs

X⁺
0.900

X⁺X⁺
0.405

X⁺X^o
0.045

X⁺Y
0.450

X^o
0.100

X⁺X^o
0.045

X^oX^o
0.005

X^oY
0.050

Assuming that the X and Y-bearing sperm are produced in an equal ratio, then 50% (0.5) of the sperm carry the Y chromosome and 50% (0.5) of the sperm carry the X chromosome. Since a small percentage of the X-bearing sperm carry the recessive color-blind gene, their numerical value is 5% (0.05). The majority of X-bearing sperm carry the dominant allele for normal vision and their numerical value is 45% (0.45. 90% (0.9) of the X-bearing eggs carry the dominant allele for normal vision, and 10% (0.1) of the eggs carry the recessive allele for color blindness. It is necessary to include these small proportions of sperm and eggs that carry color-blind alleles in order to account for the percentages of color-blind males and females in the population.
Based upon the gene frequencies of the parents in the population, all of the genotype percentages can easily be calculated by multiplying the decimal values of different sperm and eggs together. For example, 0.5% (0.005) of the population are colorblind females X^oX^o (0.05 x 0.10 = 0.005). Five percent (0.05) of the population are color-blind males X^oY (0.5 x 0.1 = 0.05). This is roughly equivalent to the percentages of color-blind females and males in the U.S. population.

For More Information, See Exercise 4 in Biology 100 Lab Manual:

Armstrong, W.P. 1988. Biology Laboratory Manual & Workbook
     Burgess International Group, Inc., Edina, Minnesota.

2. Pattern Baldness In Humans: A Sex Influenced Trait

Pattern baldness in men is a sex-influenced trait. It is not an X-linked trait like color blindness and hemophilia. Baldness is an autosomal trait and is apparently influenced by sex hormones after people reach 30 years of age or older. This condition is inherited as a simple Mendelian trait, although the biochemical mechanism is complex. There are other causes for baldness not related to simple Mendelian genetics, such as illness, severe stress, and pulling one's hair out by the roots. In men the gene is dominant, while in women it is recessive. A man needs only one allele (B) for the baldness trait to be expressed, while a bald woman must be homozygous for the trait (BB). Since a bald woman must inherit the baldness trait from her mother and father, it is less common in females. In addition, the trait typically results in women with thinning hair rather than completely bald. The gene is readily passed from mother to son because he will inherit one set of her chromosomes.
If B is the allele for baldness and b is the allele for normal hair, a bald man can be heterozygous (Bb) or homozygous bald (BB). A man with normal hair must be homozygous normal (bb). A normal woman can be homozygous normal (bb) or heterozygous (Bb). A woman who has thinning hair and a receeding hair line in later life must be homozygous bald (BB).

Sex

      Baldness

    Normal Hair

Male

BB, Bb

bb

   Female

BB

Bb, bb

If you are a man with a bald father, you are doomed to lose your hair if your father is homozygous bald (BB). If he is heterozygous (Bb), you have a 50-50 chance of inheriting his gene for normal hair (b). If you also inherit the gene for normal hair from your mother, then your genotype will be bb and your phenotype will be normal hair:

   Gametes

Sperm
B                          b

              b
  Eggs
              b
          Bb

          bb

Bb

bb

If your grandfather on the mother's side of your family is bald, then you have a 50-50 chance of inheriting this gene from your mother:

   Gametes

Sperm
b                          b

              B
  Eggs
              b
          Bb

          Bb

bb

bb

If your grandfather on the mother's side of your family is bald and your father is heterozygous bald, then you have a 75% chance of losing your hair:

   Gametes

Sperm
B                          b

              B
  Eggs
              b
          Bb

          Bb

Bb

bb

If your grandfather on the mother's side of your family is bald and your father is homozygous bald, then you have a 100% chance of losing your hair:

   Gametes

Sperm
B                          B

              B
  Eggs
              b
          BB

          BB

Bb

Bb

Baldness Thinking Question
A photo album clearly shows that both of your grandfathers were bald, but there is no other evidence of baldness in your family. What is the fractional probability that you will lose your hair? Note: This question involves that probability that your mother carries the baldness gene, and the chance that you will also inherit the gene for baldness.

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