Human Genetics Chapter 14 in the Textbook

Human Chromosomes What makes us different from other animals such as a chimpanzee or earthworm? If the genome were typed into books, you'd need 200 volumes, each 1,000 pages long, to write it all out. And yet, knowing the sequence does not mean you know where the genes are, how they are regulated, or what they do.

Human Chromosomes The size of a genome does not necessarily have anything to do with the complexity of the organism. The lowly onion has a genome that is 20 times the size of the human genome. Human genome is thought to be composed of 30 thousand genes spread out over ____ chromosomes

Human Chromosomes However the study of any genome begins with chromosomes What is a chromosome? Threadlike structure within the nucleus that contains genetic information that is passed from one generation to the next.

The gender of an individual is determined by the combination of inherited sex chromosomes. Females normally have two X chromosomes. Males normally have one X and one Y chromosome. How are the X and Y chromosomes different? Sex-Linked Inheritance

22,700,000 bases or 78 genes make up the Y chromosome, the smallest in our genome. 155 million basses or 900 genes make up the X chromosome.

Make a Punnett square to show the probability of producing a male or female offspring. – Females can only pass on a X chromosome. – Every female egg cell contains 1 X chromosome. – Males can pass on a X or Y chromosome. – Half of the sperm cells contain an X chromosome and half contain a Y chromosome. Sex-Linked Inheritance

Sex-linked Genes: genes located on a sex chromosome (X or Y) All genes found on the Y chromosome are passed directly from father to son. Males have only 1 X chromosome – All alleles on the X chromosome are expressed in males, even if they are recessive – Males are more likely to inherit a recessive trait that is found on X chromosome. – More than 100 genetic disorders are linked to the X chromosome. Sex-Linked Inheritance

An example of a sex-linked disorder is colorblindness. Genes associated with colorblindness are found only on the X chromosome 1/12 males are colorblind, 1/200 females are colorblind Draw a Punnett square to show the probability that a heterozygous normal vision female and a normal vision male would have a colorblind child. Sex-Linked Inheritance

Example: Color Blindness Test Various tests for color blindness.

Sex-Linked Traits It is possible for a female to be a carrier of an X (sex)-linked trait, but not express it. Using the picture and your prior knowledge explain why the statement above U.S. National Library of Medicine X-linked recessive, carrier mother Unaffected son Unaffected daughter Affected son Carrier daughter Unaffected father Carrier mother Carrier Affected Unaffected