12.1 X-linked Inheritance in Humans. Units of Heredity: Chromosomes and Inheritance Ch. 12. X-linked Inheritance. X-linked Inheritance

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1 Units of Heredity: Chromosomes and Inheritance Ch in Humans X-chromosomes also have non genderspecific genes Called X-linked genes Vision Blood-clotting X-linked conditions Conditions caused by malfunctioning X-linked genes Red-green color-blindness Hemophilia Figure

2 Males are XY and therefore only possess one X chromosome Because of this, X-linked conditions are more common in males Only one allele present, if that allele is dysfunctional there is no normal allele to function Females are XX Therefore possess two alleles for all these genes May have a normal allele to mask the faulty one These are recessive conditions Example A woman with a dysfunctional blood-clotting allele on one of her X chromosomes usually will be protected from hemophilia by a functional allele on her second X chromosome. Heterozygous females are carriers for these conditions Because they possess the allele, but do not phenotypically show the condition Those dysfunctional alleles may then be passed on to off-spring Homozygous recessive females will show the condition father not color-blind functional redgreen allele X sperm Y X XX XY mother not color-blind egg X XX XY nonfunctional redgreen color blind allele daughters are not color-blind one son is color-blind Figure

3 Autosomal Genetic Disorders 12.2 Autosomal Genetic Disorders Autosomal recessive disorders Disorders of genes on the autosomes (non sex chromosomes) Sickle-cell anemia Autosomal Genetic Disorders Autosomal Genetic Disorders Sickle cell anemia Recessive disorder Must be homozygous for the sickle-cell allele to suffer from the condition they must have two alleles that code for the same sickle-cell hemoglobin protein Figure

4 Autosomal Genetic Disorders Autosomal Genetic Disorders (a) Sickle-cell anemia: transmission of a recessive disorder. Dominant disorders The dysfunctional allele is dominant Therefore only one allele necessary for the disorder Huntingtons disease Fatal neurodegenerative disorder All offspring have 50% chance of inheriting the disease mother not sick S s egg S SS Ss father not sperm sick Ss ss s (b) Huntington disease: transmission of a dominant disorder. mother not sick Sickle-cell anemia is a recessive autosomal disorder; both the mother and father must carry at least one allele for the trait in order for a son or a daughter to be a sickle-cell victim. When both parents have one sickle-cell allele, there is a 25 percent chance that any given offspring will inherit the condition. 25% probability of inheriting the disorder h h egg H Hh Hh 50% probability of inheriting the disorder father sick h sperm hh hh In Huntington disease, if only a single parent has a Huntington allele there is a 50 percent chance that a son or daughter will inherit the condition. Figure 12.4 Pedigrees 12.3 Tracking Traits with Pedigrees Medical pedigrees Genetic familial histories that normally take the form of diagrams For tracking inherited diseases Allow experts to make deductions about the genetic makeup of several generations of family members 4

5 Pedigrees I?? Aa Aa A? A? female male normal II aa?? A? Aa Aa A? carrier albino III??? A? A? aa A? Figure 12.5 Polyploidy 12.4 Aberrations in Chromosomal Sets: Polyploidy Humans and many other organisms are diploid paired sets of chromosomes. In humans 46 chromosomes total 22 pairs of autosomes And either an XX chromosome pair (for females) or an XY pair (for males) 5

6 Polyploidy Polyploidy The state of having more than two sets of chromosomes Many plants are polyploid The condition is inevitably fatal for human beings Incorrect Chromosome Number: Aneuploidy Aneuploidy Aneuploidy Aneuploidy condition in which an organism has either more or fewer chromosomes than normally exist in its species full set. responsible for a large proportion of the miscarriages that occur in human pregnancies A small proportion of embryos survive aneuploidy the children who result from these embryos are born with such conditions as Down syndrome. 6

7 Aneuploidy Nondisjunction Non-disjunction The usual cause of aneuploidy in which homologous chromosomes or sister chromatids fail to separate correctly in meiosis This leads to eggs or sperm that have one too many or one too few chromosomes. Nondisjunction Aneuploidy Normal Abnormal Abnormal Nondisjunction in meiosis I Aneuploidy can come about in regular cell division (mitosis) as well as in meiosis. Nondisjunction in meiosis II % of gametes get normal number of chromosomes 100% of gametes get abnormal number of chromosomes 50% normal 50% abnormal Figure

8 Aneuploidy and Cancer Aneuploidy and Cancer Can be a cause of cancer A number of cancer researchers believe that mitotic aneuploidy is a cause rather than an effect of it Recent evidence indicates that, at the least, such aneuploidy appears prior to the initiation of some forms of cancer X Y Figure 12.9 Chromosomal Aberrations 12.6 Structural Aberrations in Chromosomes Harmful aberrations can occur within chromosomes, with many of these aberrations coming about because of mistakes in chromosomal interactions. 8

9 Chromosomal Aberrations Chromosomal Aberrations Chromosomal aberrations include: deletions inversions translocations duplications Deletion Inversion Translocation Duplication Figure