recessive

recessive Sickle-Cell Disease Sickle-cell disease affects one out of 400 African-Americans The disease is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells Symptoms include physical weakness, pain, organ damage, and even paralysis Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Distribution of.... Malaria Sickle Cell Gene

Heterozygous phenotype same as that of homo- zygous dominant PP Pp Fig. 14-UN2 Degree of dominance Description Example Complete dominance of one allele Heterozygous phenotype same as that of homo- zygous dominant PP Pp Incomplete dominance of either allele Heterozygous phenotype intermediate between the two homozygous phenotypes CRCR CRCW CWCW Codominance Heterozygotes: Both phenotypes expressed IAIB Multiple alleles In the whole population, some genes have more than two alleles ABO blood group alleles IA , IB , i Pleiotropy One gene is able to affect multiple phenotypic characters Sickle-cell disease

polydactyly Frequency of Dominant Alleles Dominant alleles are not necessarily more common in populations than recessive alleles For example, one baby out of 400 in the United States is born with extra fingers or toes                                Photos: eHand.com (the electronic textbook for hand surgery) Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Blood Typing Problem

Concept 15.2: Sex-linked genes exhibit unique patterns of inheritance In humans and some other animals, there is a chromosomal basis of sex determination Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Females are XX, and males are XY Each ovum contains an X chromosome, while a sperm may contain either an X or a Y chromosome Other animals have different methods of sex determination Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Inheritance of Sex-Linked Genes The sex chromosomes have genes for many characters unrelated to sex A gene located on either sex chromosome is called a sex-linked gene In humans, sex-linked usually refers to a gene on the larger X chromosome Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Sex-linked genes follow specific patterns of inheritance For a recessive sex-linked trait to be expressed A female needs two copies of the allele A male needs only one copy of the allele Sex-linked recessive disorders are much more common in males than in females Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Fig. 15-5 X Figure 15.5 Human sex chromosomes Y

Barr body – an inactivated X chromosome in each of a female’s diploid cells.

Correlating Behavior of a Gene’s Alleles with Behavior of a Chromosome Pair In one experiment, Morgan mated male flies with white eyes (mutant) with female flies with red eyes (wild type) The F1 generation all had red eyes The F2 generation showed the 3:1 red:white eye ratio, but only males had white eyes Morgan determined that the white-eyed mutant allele must be located on the X chromosome Morgan’s finding supported the chromosome theory of inheritance Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

The Chromosomal Basis of Sex In humans and other mammals, there are two varieties of sex chromosomes: a larger X chromosome and a smaller Y chromosome Only the ends of the Y chromosome have regions that are homologous with the X chromosome The SRY gene on the Y chromosome codes for the development of testes Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Fig. 26.14b Sex-linked recessive

Hemophilia Sex-linked recessive pingrybiology.pbwiki.com www.kelleycom.com Hemophilia is a rare, inherited bleeding disorder in which blood doesn’t clot normally. People with hemophilia may bleed for a longer time than others after an injury. Individuals with the disorder may bleed internally, especially in knees, ankles, and elbows. This bleeding can damage organs or tissues and, sometimes, be fatal.

Hemophilia & The Royal Family The European Royal Family                                                                                                                                                                         back

Photo: www.freewebs.com/georgeromanov/Foto%201913.jpg

Color Blindness some forms are Sex-linked

Fig. 26.18