1. 14-1 Human Heredity
Photo credit: Richard Hutchings/Photo Researchers, Inc.

2. Human Chromosomes 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.

3. 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.

4. 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.

5. Human Chromosomes
How is sex determined?

6. Human Chromosomes
All human egg cells carry a single X chromosome (23,X).
Half of all sperm cells carry an X chromosome (23,X) and half carry a Y chromosome (23,Y).
About half of the zygotes will be 46,XX (female) and half will be 46,XY (male).

7. 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).

8. Human Traits Human Traits
In order to apply Mendelian genetics to humans, biologists must identify an inherited trait controlled by a single gene.
They must establish that the trait is inherited and not the result of environmental influences.
They have to study how the trait is passed from one generation to the next.

9. Human Traits Pedigree Charts
A pedigree chart shows the relationships within a family.
Genetic counselors analyze pedigree charts to infer the genotypes of family members.

10. Human Traits A square represents a male.
A horizontal line connecting a male and a female represents a marriage.
A circle represents
a female.
A vertical line and a bracket connect the parents to their children.
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.

11. Human Traits Genes and the Environment
Some obvious human traits are almost impossible to associate with single genes.
Traits, such as the shape of your eyes or ears, are polygenic, meaning they are controlled by many genes.
Many of your personal traits are only partly governed by genetics.

12. Human Genes Blood Group Genes
Human blood comes in a variety of genetically determined blood groups.
The best known are the ABO blood groups and the Rh blood groups.

13. Human Genes ABO blood group
There are three alleles for this gene, IA, IB, and i.
Alleles IA and IB are codominant.

14. Human Genes
Individuals with alleles IA and IB are blood type AB.

15. Human Genes The i allele is recessive.
Individuals with alleles IAIA or IAi are blood type A.

16. Human Genes
Individuals with IBIB or IBi alleles are type B.

17. Human Genes
Individuals who are homozygous for the i allele are said to have blood type O.

18. Human Genes Recessive Alleles
The presence of a normal, functioning gene is revealed only when an abnormal or nonfunctioning allele affects the phenotype.
Many disorders are caused by autosomal recessive alleles.

19. Human Genes
This table shows the major symptoms of some well-known genetic disorders.

20. Human Genes Dominant Alleles
The effects of a dominant allele are expressed even when the recessive allele is present.
Two examples of genetic disorders caused by autosomal dominant alleles are achondroplasia and Huntington disease.

21. Human Genes
This table shows the major symptoms of some well-known genetic disorders.

22. Human Genes Codominant Alleles
Sickle cell disease is a serious disorder caused by a codominant allele.
Sickle cell is found in about 1 out of 500 African Americans.

23. Human Genes

24. 14.2
Genes located on these on the chromosomes X and Y are called sex-linked genes.
Most are found on the X chromosome and very few on the Y chromosome.

25. 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.

26. Chromosomal Disorders
What problems does nondisjunction cause?
The most common error in meiosis occurs
when homologous chromosomes fail to separate.
This is known as nondisjunction, which means, “not coming apart.”

27. If nondisjunction occurs, abnormal numbers of chromosomes may find their way into gametes, and a disorder of chromosome numbers may result.

28. NonDisjunction
Nondisjunction causes gametes to have abnormal numbers of chromosomes. The result of nondisjunction may be a chromosome disorder such as Down syndrome.

29. Chromosomal Disorders
Down Syndrome Karyotype
Down syndrome produces mild to severe mental retardation.
It is characterized by:
increased susceptibility to many diseases
higher frequency of some birth defects
This karyotype is from a person with Down syndrome. Down syndrome causes mental retardation and various physical problems. People with Down syndrome can, however, lead active, happy lives. Photo credit: ©Dr. Dennis Kunkel/CNRI/Phototake