Chapter 11 – Complex Inheritance and Human Heredity Unit 4B - Genetics Chapter 11 – Complex Inheritance and Human Heredity

Basic Patterns of Inheritance 11.1 Essential Questions How can genetic patterns be analyzed to determine dominant of recessive inheritance patterns? What are examples of dominant and recessive disorders? How can human pedigrees be constructed from genetic information?

Recessive Genetic Disorders A recessive trait is expressed when there is no dominant trait to hide it. A person who is heterozygous for a recessive trait is called a carrier – they carry the trait but don’t express it. Examples of recessive disorders are: Cystic fibrosis, albinism, galactosemia, Tay-Sachs disease.

Dominant Genetic Disorders In dominant genetic disorders, you either have it or you don’t. There are no carriers. Examples are: Huntington’s disease and achondroplasia.

What is a Pedigree? Pedigrees are like a family tree. They trace a trait through several generations. It is a way to graphically represent genetics. It uses symbols to identify characteristics.

The Symbols Used…

Labeling the Generations Generations are shown with Roman Numerals (I, II, III, IV & V). Individuals are 1, 2, 3, 4, etc. The oldest generation is at the top; youngest at the bottom.

Complex Patterns of Inheritance 11.2 Essential Questions What are the differences between various complex inheritance patterns? How can sex-linked inheritance patterns be analyzed? How can the environment influence the phenotype of an organism?

Incomplete Dominance Neither gene is dominant; Both genes have SOME control, so a third phenotype is present that “appears” to be a blend. CRCR = Red Flower CrCr = White Flower CRCr = Pink Flower

Co-Dominance Both genes are dominant and expressed equally. B = Black Chicken W = White Chicken BW = Checkered Chicken (not gray!)

Codominance in Humans – Sickle Cell Anemia Homozygous Form – most severe. Causes malformed blood cells that can’t carry oxygen. Causes tissue damage and pain. Patients are treated with medicine and transfusions. Heterozygous Form – mild symptoms because sickled and normal hemoglobin (blood cells) are formed. These patients lead mostly normal lives.

Multiple Alleles Traits controlled by more than one allele are called multiple alleles. Skin color can involve 3-4 genes. Blood type is the most common example:

Epistasis One allele can hide the effect of another allele. In labrador retreivers, the dominant allele E determines whether fur will have dark pigment. The dominant allele B determines how dark the pigment in the coat will be.

Epistasis (continued) EEbb or Eebb – chocolate EEBB, EeBB, EEBb, or EeBb - black eebb, eeBb, and eeBB - yellow

Is It a Boy or a Girl? Humans have 23 pairs of chromosomes. 22 are somatic (body cells); they are called autosomes. 1 pair is sex chromosomes and it determines your gender. XX – female XY - male

Sex-Linked Inheritance Traits controlled by genes located on the sex chromosomes are called “sex-linked” traits. Females can carry the X-linked traits and have them. Males cannot be carriers of X-linked traits; they can only have the trait. Ex: The top part of the Y is missing. The sex-linked traits are on the upper portion of the X chromosome.

Common Sex-Linked Disorders Red-green colorblindness Hemophilia Duchenne’s Muscular Dystrophy Male pattern baldness

Sex-Linked Traits in a Pedigree Females carry the trait, but are not affected. Males have the trait and are not ever a carrier. Can you figure out the genotypes of this generation?

Practice #1 A man and woman marry. They have five children, 2 girls and 3 boys. The mother is a carrier of hemophilia, an X-linked disorder. She passes the gene on to two of the boys who died in childhood and one of the daughters is also a carrier. Both daughters marry men without hemophilia and have 3 children (2 boys and a girl). The carrier daughter has one son with hemophilia. One of the non-carrier daughter’s sons marries a woman who is a carrier and they have twin daughters. What is the percent chance that each daughter will also be a carrier?

Practice #2 The great-great maternal grandmother of a boy was a carrier for color-blindness, an X-linked disorder. His great uncle on his mother’s side was colorblind but this great uncle’s father was unaffected. The boy’s mother has 2 brothers (1 colorblind, 1 unaffected) and 1 sister (unaffected). The boy’s grandmother on his mother’s side had 1 brother who was colorblind and 3 sisters. Two of these sisters were unaffected and one was a carrier. The boy’s great grandmother on his mother’s side had 4 sisters. The boy has one unaffected sister and he is colorblind. What is the probability of the boy’s sons being colorblind if he marries a non-carrier?

Homework #3 An unaffected man marries a woman who is a carrier for Duchenne Muscular Dystrophy, which is attributed to an X-linked gene. They have four children, one with Duchenne, one carrier daughter and a daughter and son who are unaffected. The child with Duchenne Muscular Dystrophy dies in childhood. The carrier daughter marries and has three children of her own, two of which are carriers and one of which is unaffected. What is the most likely sex of these two carrier children given the fact that they are unaffected by the X-linked gene?

Polygenic Traits Many traits are controlled by the interaction of multiple pairs of genes. Skin color, height, eye color, and fingerprint patterns are polygenic. It is based on the number of dominant alleles between all the genes.

Polygenetics and skin color…

Influence of the Environment Internal influences like hormones and age can affect gene expression. External things like temperature, light, nutrition, chemicals, and infectious agents can also be an influence. Example: Himalayan rabbit

Meet Tallulah/Louie!

Chromosomes & Human Heredity 11.3 Essential Questions How are karyotypes used to study genetic disorders? What is the role of telomeres? How is nondisjunction related to Down Syndrome and other abnormal chromosome numbers? What are the benefits and risks of diagnostic fetal testing?

Karyotypes Scientists can look at whole chromosomes. They are stained to show bands/stripes. The best time to observe them is during metaphase of mitosis. They are arranged in order of size from largest to smallest. There are 22 autosomes and 1 pair of sex chromosomes.

A Human Karyotype

Telomeres A telomere is a protective cap on the ends of a chromosome. They might be linked to cancer and aging.

Nondisjunction The chromosomes don’t separate properly during meiosis. Can result in extra chromosomes or missing chromosomes in a gamete. 1 chromosome – monsomy (-1) 3 chromosomes – trisomy (+1) Can occur in sex chromosomes also.

Changes in Chromosome Numbers These can cause several medical conditions in humans: Down Syndrome - trisomy of chromosome 21 Cri Du Chat Syndrome – a piece of chromosome 5 is missing Patau Syndrome - trisomy of chromosome 13 Edward Syndrome - trisomy of chromosome 18 Klinefelter Syndrome - an extra X chromosome in males Turner Syndrome - only one X chromosome present in females XYY Syndrome - an extra Y chromosome in males Triple X Syndrome - an extra X chromosome in females

Fetal Testing Couples with known genetic disorders in their families or who are older can be tested. Amniocentesis, Fetal Blood Sampling, and Chorionic Villus sampling are all ways to test a fetus. There are benefits and risks to each type of test.