1. Applied Genetics and Pedigrees

2. Copyright Pearson Prentice Hall
Incomplete Dominance
When one allele is not completely dominant over another it is called incomplete dominance.
In incomplete dominance, the heterozygous phenotype is between the two homozygous phenotypes.
Copyright Pearson Prentice Hall

3. Beyond Dominant and Recessive AllelesRR
A cross between red (RR) and white (WW) four o’clock plants produces pink-colored flowers (RW). WW
Some alleles are neither dominant nor recessive. In four o’clock plants, for example, the alleles for red and white flowers show incomplete dominance. Heterozygous (RW) plants have pink flowers—a mix of red and white coloring.
Copyright Pearson Prentice Hall

4. Copyright Pearson Prentice Hall
Codominance
In codominance, both alleles contribute to the phenotype.
In certain varieties of chicken, the allele for black feathers is codominant with the allele for white feathers.
Heterozygous chickens are speckled with both black and white feathers. The black and white colors do not blend to form a new color, but appear separately.
Copyright Pearson Prentice Hall

5. Copyright Pearson Prentice Hall
Multiple Alleles
Genes that are controlled by more than two alleles are said to have multiple alleles.
An individual can’t have more than two alleles. However, more than two possible alleles can exist in a population.
Copyright Pearson Prentice Hall

7. Copyright Pearson Prentice Hall
Human Chromosomes
Cell biologists analyze chromosomes by looking at karyotypes.
Cells are photographed during mitosis. Scientists then cut out the chromosomes from the photographs and group them together in pairs.
A picture of chromosomes arranged in this way is known as a karyotype.
Copyright Pearson Prentice Hall

8. Copyright Pearson Prentice Hall
Human Chromosomes
Human Karyotype
These human chromosomes have been cut out of a photograph and arranged to form a karyotype. Photo credit: ©CNRI/Science Photo Library/Photo Researchers, Inc.
Copyright Pearson Prentice Hall

9. Copyright Pearson Prentice Hall
Human Chromosomes
Two of the 46 human chromosomes are known as sex chromosomes, because they determine an individual's sex.
Females have two copies of an X chromosome.
Males have one X chromosome and one Y chromosome.
The remaining 44 chromosomes are known as autosomal chromosomes, or autosomes.
Copyright Pearson Prentice Hall

10. Copyright Pearson Prentice Hall
Human Chromosomes
How is sex determined?
Copyright Pearson Prentice Hall

11. Copyright Pearson Prentice Hall
Human Chromosomes
Males and females are born in a roughly 50 : 50 ratio because of the way in which sex chromosomes segregate during meiosis.
In humans, egg cells contain a single X chromosome. Sperm cells contain either one X chromosome or one Y chromosome. In a population, approximately half of the zygotes are XX (female) and half are XY (male).
Copyright Pearson Prentice Hall

12. Copyright Pearson Prentice Hall
Sex-Linked Genes
The X chromosome and the Y chromosomes determine sex.
Genes located on these chromosomes are called sex-linked genes.
More than 100 sex-linked genetic disorders have now been mapped to the X chromosome.
Copyright Pearson Prentice Hall

13. Copyright Pearson Prentice Hall
X Chromosome
Duchenne muscular dystrophy
The Y chromosome is much smaller than the X chromosome and appears to contain only a few genes.
Melanoma
X-inactivation center
X-linked severe combined immunodeficiency (SCID)
Colorblindness
Hemophilia
Genes on X and Y chromosomes, such as those shown in the diagrams, are called sex-linked genes.
Y Chromosome
Testis-determining
factor
Copyright Pearson Prentice Hall

14. Copyright Pearson Prentice Hall
Sex-Linked Genes
Why are sex-linked disorders more common in males than in females?
Copyright Pearson Prentice Hall

15. Copyright Pearson Prentice Hall
Sex-Linked Genes
For a recessive allele to be expressed in females, there must be two copies of the allele, one on each of the two X chromosomes.
Males have just one X chromosome. Thus, all X-linked alleles are expressed in males, even if they are recessive.
Copyright Pearson Prentice Hall

16. Copyright Pearson Prentice Hall
Sex-Linked Genes
Colorblindness
Three human genes associated with color vision are located on the X chromosome.
In males, a defective version of any one of these genes produces colorblindness.
Copyright Pearson Prentice Hall

17. Copyright Pearson Prentice Hall
Sex-Linked Genes
Hemophilia
The X chromosome also carries genes that help control blood clotting. A recessive allele in either of these two genes may produce hemophilia.
In hemophilia, a protein necessary for normal blood clotting is missing.
Hemophiliacs can bleed to death from cuts and may suffer internal bleeding if bruised.
Copyright Pearson Prentice Hall

18. Copyright Pearson Prentice Hall
Pedigree Charts
A pedigree chart shows the relationships within a family.
Genetic counselors analyze pedigree charts to infer the genotypes of family members.
Copyright Pearson Prentice Hall

19. Copyright Pearson Prentice Hall
A circle represents
a female.
A square represents
a male.
A vertical line and a bracket connect the parents to their children.
A horizontal line connecting a male and a female represents a marriage.
A circle or square that is not shaded indicates that a person does not express the trait.
A shaded circle or square indicates that a person expresses the trait.
This drawing shows what the symbols in a pedigree represent.
Copyright Pearson Prentice Hall

20. What is a carrier?
A carrier is represented by a half-shaded circle or square. They are heterozygous – they have the recessive gene but it does not show.