Chapter 11, Complex Genetic Patterns

Recessive Genetic Disorders In chapter 10 we discussed that in order for a recessive trait to be expressed, an individual must be homozygous recessive for that trait. Recessive genetic disorders are disorders that are caused by recessive alleles when a person is homozygous recessive for that trait. A person who is heterozygous for a trait does not express the trait but can pass the trait to their offspring. They are called carriers.

Recessive Genetic Disorders

Recessive Genetic Disorders Cystic Fibrosis – Causes a thick, sticky mucus to build up in the lungs and digestive tract. Albinism – No color in the skin, eyes, and hair. Galactosemia – Inability of the body to break down the simple sugar galactose. Tay-Sachs disease – Deadly disease of the nervous system that is most common in people of Jewish descent.

Albinism Mother of five, with three albino children

Dominant Genetic Disorders Caused by dominant genes. If only one parent has one allele (heterozygous), 50% of the children will inherit the disease.

Dominant Genetic Disorders Huntington's disease – Causes nerve cells in the brain to degenerate, causing a gradual loss of brain function. Occurs most commonly in people ages 30 – 50 years old. Achondroplasia – A disorder that affects the growth of bones and causes dwarfism. Polydactyly – A condition resulting in an extra number of fingers and toes.

Polydactyly

Pedigrees Geneticists use diagrams that trace inheritance of particular traits through several generations called pedigrees. A pedigree uses symbols to illustrate different meanings. Males are represented by a square. Females are represented by a circle. Usually someone who expresses a trait is dark. Usually someone without the trait is light. A carrier is half dark and half light.

Pedigrees Horizontal lines between symbols represent parents of the offspring in the lines below them. Offspring are ordered from first to last. Roman numerals represent generations.

Albinism and Polydactyly Pedigrees

Incomplete Dominance Remember from chapter 10 we learned that if an organism's genotype is homozygous dominant (TT) or heterozygous (Tt) the dominant allele is expressed. Only when an individual organism's genotype is homozygous recessive (tt) is the recessive allele expressed. In incomplete dominance, when an organism's genotype is heterozygous (Tt), an intermediate phenotype is expressed.

Incomplete Dominance When Red (RR) and White (rr) snapdragons are crossed, the heterzygous offspring are all pink (Rr).

Codominance Codominance is when two alleles can be dominant at the same time. In coat color for cows, you can have red cows (RR), white cows (WW), and roan cows (RW). Roan cows have both red and white hairs. In chickens, you can have chickens with black feathers (BB), chickens with white feathers (WW), and chickens with black and white feathers (BW), but not gray feathers.

Codominance (Roan Cattle)

Multiple Alleles Some genes are controlled by more than just two different alleles (dominant and recessive). Blood types in humans is determined by multiple alleles. Humans can have either A blood, B blood, AB blood (codominant), or O blood (neither A or B blood). Multiple alleles can also express a hierarchy of dominance. For example in rabbit coat color, Full coat color is dominant to Chinchilla, Chinchilla is dominant to Himalayan, and Himalayan is dominant to Albino.

Multiple Alleles and Dominance Hierarchy

Epistasis Some genes can be altered or modified by the effects of one or more other genes. This effect is called epistasis. A good example of this is coat color in Labrador retrievers. In this case, the dominant allele (E) determines whether or not the fur is dark. A second gene (B) determines how dark the coat color will be.

Sex Determination Humans have 46 chromosomes. 2 of those 46 chromosomes are called sex chromosomes. The other 44 chromosomes are called autosomes. Most of your chromosomes are autosomes are chromosomes that determine your traits. Sex chromosomes can determine traits as well, but they also determine gender or sex.

Sex Determination in Chromosomes Sex chromosomes can either be X or Y. If an individual's sex chromosomes are both X chromosomes, that person will be a female. If an individual's sex chromosomes are X and Y, that person will be a male.

Sex-Linked Traits Traits controlled by genes on the sex chromosomes are called sex-linked traits. Because males have only one X chromosome, they are affected by recessive traits found on the X chromosome more than females. Females are less likely to express a recessive trait on an X chromosome, because the other X chromosome may have a dominant allele.

Are you Color Blind?

How are Sex-linked traits inherited? This would be the result from a cross between a female carrier (XBXb) for color blindness and a normal male (XBY). ½ of the children would be normal ½ would either be color blind or be carriers

Sex-linked traits Hemophilia is another sex-linked trait that causes delayed blood clotting. Most sex-linked traits like color blindness and hemophilia affect males more than females. Calico cats are always females because the gene for coat color is on the X chromosome. If a female is heterozygous, she will be a calico cat. Male pattern baldness is technically an autosomal trait, but it acts like a sex-linked trait. Eye color in Drosophila Fruit flies is a sex-linked trait.

Calico Cats Males can either be black or orange. Females can be black, orange, or calico. This is caused by one of the X chromosomes becoming inactive in female cats.

Queen Victoria's Pedigree (Hemophilia)

Male Pattern Baldness The gene for baldness is recessive in females, but dominant in males. Males can either be heterozygous or homozygous dominant. Females must be homozygous recessive to be bald.

Drosophila Fruit Flies Red-eyes are dominant to white-eyes in fruit flies. The gene for eye color in fruit flies is found on the X chromosome. Fruit flies make good specimens for studying sex-linked traits.

Polygenic Traits Genes that are controlled by multiple genes at the same time are called polygenic traits One characteristic of polygenic traits is that there tends to be a higher frequency of intermediate forms of a trait. When graphed, the frequency of phenotypes for polygenic traits appears as a bell-shaped curve. Some examples of polygenic traits are skin color, eye color, and height.

Polygenic Traits (Height)

Polygenic Traits (Skin Color)

Polygenic Traits (Eye Color)

What are Karyotypes? Some genetic disorders can be caused by the presence or absence of chromosomes. A Karyotype is a photograph of a person's chromosomes arranged in order. They are arranged from largest to smallest. Homologous pairs are arranged together. Once arranged, karyotypes can reveal genetic disorders by looking for patterns.

Normal Karyotype

Nondisjunction If chromosomes fail to separate properly during meiosis, the gametes (sex cells) will not end up with the right number of chromosomes. Later on during fertilization if the egg or sperm contains one of these abnormal gametes, a nondisjunction can occur. If a zygote ends up with one extra chromosome, this results in a trisomy (2n + 1) If a zygote ends up with one less chromosome, this results in a monosomy (2n - 1)

Nondisjunction

Disorders caused by Nondisjunction Down syndrome – result of an extra chromosome number 21 (often called trisomy 21). Symptoms include short stature, heart defects, and mental disability, increased risk of infection. About 1 in 800 children born in the U.S. have Down syndrome. The frequency of children being born with Down syndrome increase with the age of the mother. Mothers over 45 have about a 6% chance of giving birth to a child with Down syndrome.

Down Syndrome Karyotype

Other Disorders caused by Nondisjunction Turner's syndrome – (XO) Women with only one x chromosome. Symptoms include short height, webbed neck, lack of underarm and pubic hair, and underdeveloped ovaries. Klinefelter's syndrome – (XXY) Symptoms in males include tall height, low IQ scores, speech and language difficulty, sterility. Patau syndrome – (trisomy 13) Heart defects, abnormalities of the eyes, ears, brain and spinal cord, cleft palate and/or lip, small head, low IQ scores, additional fingers and toes, 95% mortality in the first year of life, children rarely live past a few months. Edward’s Syndrome – (trisomy 18) Slow growth before birth, low birth weight, abnormal organ growth, heart defects, small head, 90- 95% mortality rate during the first year.