Complex Inheritance and Human Heredity Chapter 11

Let’s Review! We inherit traits from our parents, we get half of our genes from mom, and half from dad Alleles are either recessive (lowercase) or dominant (uppercase) Homozygous recessive, homozygous dominant, heterozygous

Genetic Disorders An abnormal condition that a person inherits through their genes Can be caused by either changes in DNA, or an extra or missing chromosome Genetic disorders can be recessive or dominant

Recessive Disorders Two recessive alleles, one from each parent If an individual is heterozygous, they are a carrier (this means they could pass it on even if they don’t have it!) BB – Normal, Bb – Normal, but a carrier, bb – Disease Often skips a generation

Recessive Disorders Cystic Fibrosis: Affects cell membrane protein, water cannot diffuse from cells body produces an abnormally thick mucus in the lungs Chromosome 7 Medication, strict diet, enzyme replacement Death is usually caused by lung complications

Recessive Disorders Tay-Sachs: Absence of enzyme that breaks down fatty substances Chromosome 15 Buildup of fatty acids in brain, cherry red spot on back of eye Causes deafness, blindness, severe retardation, and death usually by age 4 Most carriers choose to avoid birth because there is no cure Brain on top is normal, brain on bottom has lipid accumulation (is “clogged”)

Recessive Disorders Albinism: altered genes resulting in the absence of the skin pigment melanin in hair and eyes White hair Very pale skin Pink pupils

Recessive Disorders Galactosemia: characterized by the inability of the body to digest galactose. People with this disorder should avoid milk products

Dominant Disorders Caused by dominant alleles Those who do not have the disorder are homozygous recessive. BB – Disease, Bb – Disease, bb – Normal THERE ARE NO CARRIERS OF DOMINANT DISORDERS! ONE DOMINANT ALLELE MEANS YOU HAVE THE DISORDER

Dominant Disorders Huntington’s Disease: Affects the nervous system, deterioration of brain tissue, usually begins between age 30 and 40 Chromosome 4 No cure, but have medications to cope with symptoms People usually die 15-20 years after onset of degeneration

Dominant Disorders Achondroplasia: Causes small body size and limbs that are comparatively short, most common type of dwarfism Chromosome 4 75% born to parents of average size When this happens it is due to a gene mutation

Complex Inheritance Sometimes the laws and principles of genetics established by Gregor Mendel do not always explain the inheritance patterns seen in an organism. Codominance: Both alleles completely expressed in heterozygous condition.

Complex Inheritance Sickle Cell Anemia: Abnormal hemoglobin causes a person’s red blood cells to be sickle shaped, causing clots and blockages, they also carry less oxygen Alleles for the disease are codominant rr – all sickle shaped cells, Rr – mixture of both, RR – normal shaped cells People who are heterozygous for sickle cell also have a higher resistance to malaria Consequently, fewer people with sickle cell die from malaria, so the gene continues to pass

Complex Inheritance Incomplete Dominance: Neither allele is completely dominant nor completely recessive Heterozygous phenotype is a blend of the two homozygous phenotypes a mix in physical appearance between dominant and recessive

Codominant or Incomplete?

1 gene, more than 2 alleles Complex Inheritance Multiple Alleles: When there are 3 or 4 alleles that code for a single trait Blood Groups This doesn’t mean a person has more than 2 alleles for the trait, just that more than 2 exist in the population 1 gene, more than 2 alleles

Complex Inheritance 3 forms of the allele IA – type A, IB – type B, i – type O i is recessive to IA and IB IA and IB are codominant IAIB – type AB

Complex Inheritance Heterozygous Genotype IAi – Type A IBi – Type B IAIB – type AB Homozygous Genotype IAIA – Type A IBIB – Type B ii – Type O

Complex Inheritance Epistasis: When the one allele hides the effects of another A dog’s coat color Polygenic Traits: traits that result from the interaction of multiple gene pairs Skin color, height, eye color, possibly intelligence

Sex-Linked Traits Genes that are found on the X chromosome – X-linked Typically recessive, IF dominant, an affected father will pass it to all of his daughters Recessive expressed more in males because they only have one X (XY) compared to females (XX)

Sex-linked Traits Hemophilia Recessive sex-linked disorder Takes a long time for blood to clot (body lacks proteins involved in clotting) Very rare in females because she would need both X’s with recessive alleles.

Patricia is a healthy carrier of hemophilia and Sam is completely healthy. Complete the Punnett square below. Mom’s genotype: Dad’s genotype: What is the probability of getting: Daughter with hemophilia? Son with hemophilia? Carrier? Healthy child?

Sex-linked Traits Red-Green Colorblindness Recessive sex-inked Male needs 1 copy, female needs two copies, so it is very rare to find a color blind female.

Judy is homozygous recessive for colorblindness and Dennis is healthy Judy is homozygous recessive for colorblindness and Dennis is healthy. Complete their Punnett square below. Mom’s genotype: Dad’s genotype: What is the probability of getting: Colorblind daughter? Color blind son? Carrier? Heterozygous child?

Sex-linked Traits Baldness is X-linked Recessive Grandpa = XbY Grandma = XBXB daughter = XBXb marries Male = XBY Sons? 50% XB (normal) 50% Xb (bald) This is why men should look to their mom’s dad for probability of baldness. But beware grandma could be a carrier too!

Environmental Factors The environment can have an influence on phenotypes Toxic agents, Diet and exercise, Sunlight and water, Temperature, Medications Conditions can cause a gene to shut down or turn on Twins have identical genes, scientists conclude that twins with different phenotypes are influenced by the environment

Pedigrees Pedigree: Diagram that traces the inheritance of a particular trait through several generations of the same family Allows us to use family trees and affected individuals to predict the risk of disease in future offspring

Pedigrees Individuals are in birth order from left to right (oldest at left, youngest on right) Individuals are numbered, Generations are numbered with roman numerals When referring to an individual, use the generation number, then the individual number II-4

I II III Example of a Pedigree Do any disorders run in this family?? Grandparents Grandparents I Parents II Aunts, Uncles Aunts, Uncles III Brother You Do any disorders run in this family??

Pedigrees From a pedigree, you can tell whether a family carries an autosomal dominant/recessive disorder, or sex-linked disorder Autosomal or Sex-linked: Autosomal will show in both sexes equally Sex-linked will show mostly in males Dominant or Recessive: Dominant – every individual with trait has parent with trait Recessive – individual with trait has parent without trait

Karyotype Karyotype: an image that shows homologous chromosomes arranged in decreasing size The staining bands mark identical places on the homologous chromosomes

What’s different?

Nondisjunction Cell division during which sister chromatids fail to separate properly If this occurs during Meiosis I or II then the resulting gametes will not have the correct number of chromosomes Nondisjunction occurs in both autosomes (body cells) and in gametes Trisomic: one extra chromosome (only 3 trisomies that result in survival after birth) Monosomic: one missing chromosome (usually lethal, except in Turner’s Syndrome)

Down Syndrome Chromosome 21 Trisomy Can be moderate or severe Short stature, heart defects, and mental disability Correlated with mother’s age, can be from nondisjunction of father’s chromosome 21

Nondisjunction Autosomal Edwards’s Syndrome - Trisomy 18: almost every organ system affected 1:10,000 live births. Children with full Trisomy 18 generally do not live more than a few months. Patau Syndrome - Trisomy 13: serious eye, brain, circulatory defects as well as cleft palate. 1:5000 live births. Children rarely live more than a few months Click above for Choosing Thomas.

Nondisjunction Sex Chromosomes Klinefelter Syndrome – XXY: Male sex organs; unusually small testes, sterile. Breast enlargement and other feminine body characteristics. Normal intelligence. Trisomy X – XXX: 1:1000 live births - healthy and fertile - usually cannot be distinguished from normal female Monosomy X – Turner’s Syndrome: 1:5000 live births; the only viable monosomy in humans - women with Turner's have only 45 chromosomes!!! XO individuals are genetically female, however, they do not mature sexually during puberty and are sterile. Short stature and normal intelligence. (98% of these fetuses die before birth)