Complex Inheritance and Human Heredity Chapter 11

I. Recessive Genetic Disorders A recessive genetic disorder – the disorder is expressed when an individual is homozygous recessive for the trait. Carrier – carriers are heterozygous for the disorder so carry the gene but do not express the disorder because it is masked by the dominant alele.

1. Alkaptonuria  This disorder results in black urine!  Caused by an enzyme deficiency.  Acid excreted into the urine due to enzyme deficiency.  Discovered by Dr. Archibald Garrod (U.K.)  He determined that this disorder runs in families and is expressed in those who are homozygous recessive.

2. Albinism  Genes don’t produce enough pigment – called melanin.  No color in skin, eyes, hair.  Pink pupils, skin susceptible to UV damage.  1 in 17,000 (in the U.S.)

3. Cystic Fibrosis  Excess mucous in the lungs and digestive tract  Very susceptible to infections  Usually don’t live past childhood  1 in 3500 (in the U.S.)

4. Tay-Sachs Disease  Fats accumulate in the nerve cells in the brain  Brain deteriorates  Blindness, deafness, and muscle atrophy sets in  Patients usually die by age 5  Affects 1 in 2500 (in the U.S.) – usually people of Jewish descent

Tay-Sachs Disease:

5. Galactosemia  Lactose intolerance  Inability of the body to digest galactose. lactose = galactose + glucose  Galactose must be broken down into glucose by GALT enzyme  Those who lack or have defective GALT cannot digest galactose and should avoid milk products.  Affects 1 in 50,000 to 70,000 (in the U.S.)

II. Dominant Genetic Disorders  Some disorders are caused by dominant alleles.  Expressed when at least one dominant allele is present.  Those who do not have the disorder are homozygous recessive for the trait.

1. Achondroplasia  Dwarfism  Affects 1 in 25,000 (in U.S.)  Cartilage in bones of arms and legs slowly hardens resulting in shorter than normal arms, legs, and fingers.

2. Huntington’s Disease  Appears between the ages of 30 and 50 years old  Affects the nervous system  Causes loss of brain function, muscle spasms, and emotional disturbances  Affects 1 in 10,000 (in the U.S.)

Codominance and Multiple Alleles: ABO blood groups are examples of both. Blood Type Phenotype Possible Genotype A I A I A or I A i B I B I B or I B i AB I A I B (codominance) Oii Multiple Alleles - more than two possible alleles for a specific trait (the blood type gene - I A, I B and i). Codominance in AB blood type as both I A and I B are expressed simultaneously.

Sex Determination  46 chromosomes in humans (23 pairs)  Autosomes (22 pairs)  Sex chromosomes (1pair) XX = female XY = male XX = female XY = male  Whose gamete determines the gender of the offspring?

Dosage Compensation  X Chromosome Is larger than the Y Is larger than the Y Carries genes necessary for the development of both females and males. Carries genes necessary for the development of both females and males.  Y Chromosome mainly carries genes for male traits. mainly carries genes for male traits.  Dosage Compensation (x-inactivation) one X in females is inactive. This is a random event that occurs in all mammals. one X in females is inactive. This is a random event that occurs in all mammals.

Example: The coat color of a calico cat is the result of the random inactivation of one of the X chromosomes. In some cells, the X chromosome that was inherited from the mother is expressed, and in others, the X chromosome inherited from the father is expressed.

Sex-Linked Traits  Traits controlled by genes located on the X chromosome  Males affected by recessive X-linked traits more often than females. WHY?

Hemophilia – delayed clotting of blood. A recessive sex-linked disorder A recessive sex-linked disorder Affects more men than women Affects more men than women Hemophiliacs bruise easily. A small cut can result in a huge loss of blood which can cause death. Hemophiliacs bruise easily. A small cut can result in a huge loss of blood which can cause death. See Queen Victoria’s Pedigree on page 308 in your text book. See Queen Victoria’s Pedigree on page 308 in your text book.

Pedigree  A diagram that traces the inheritance of a particular trait through several generations. AA A A = MALE = FEMALE

Pedigrees…… - are used to determine genotypes from observed phenotypes. - help genetic counselors determine whether inheritance patterns are dominant, recessive, or sex-linked. - can be used to predict disorders in future offspring.

Pedigrees can be used to trace genetic disorders: shaded = affected individual

Hemophelia Pedigree

Complex Patterns of Inheritance A. Incomplete Dominance – when the heterozygous phenotype is an intermediate phenotype between the two homozygous phenotypes.

B. Codominance – both alleles are expressed in the heterozygous condition.  Sickle-cell Anemia Red blood cells are Red blood cells are abnormally shaped. Causes blood clots Causes blood clots and slow blood circulation.

 All calico cats are female because coat color is on the X chromosome.  Since females have 2 X chromosomes, in all cells one of the X chromosomes is turned off = orange and black spots.  Males have just 1 X chromosome so they can only have one color of spots.

Sex-Linked Disorders  1. Colorblindness – a recessive x-linked trait. Affects 1 in 10 males (males have one X chromosome so all traits on X chromosome are expressed). Affects 1 in 10 males (males have one X chromosome so all traits on X chromosome are expressed).

Polygenic Traits  Phenotypic traits that arise from the interaction of multiple pairs of genes. Skin color Skin color Height Height Eye color Eye color Fingerprint patterns Fingerprint patterns

Environmental Influences  Can have an effect on phenotype. Heart disease – diet and exercise Heart disease – diet and exercise Plant growth and survival – sunlight and water Plant growth and survival – sunlight and water A Siamese cat’s fur color!! – temperature affects gene expression A Siamese cat’s fur color!! – temperature affects gene expression

Nondisjunction: A. Normal Meiosis B. Failure of homologous chromosome pair to separate during Meiosis I C. Failure of sister chromatids to separate during Meiosis II

Chromosome Disorder  Down’s syndrome = extra copy of chromosome 21  Called trisomy 21  Chromosomes fail to separate during meiosis  Affects 1 in 800 babies

Trisomy 21 – Down’s Syndrome

Normal karyotype:

Sex Chromosome Disorders  Only one X chromosome = sterile female because sex organs don’t develop (Turner’s syndrome)  Extra X chromosomes (xxy or xxxy) result in sterile males  Cannot have just a y – would be fatal