Biology Chapter 20 Patterns of Genetic Inheritance Notes
What is genotypic ratio?
-the number of offspring with the same genotype
What is phenotypic ratio?
-the number of offspring with the same outward appearance
What are polygenic traits?
- 2 or more sets of alleles govern 1 trait - Each dominant allele codes for a product, so these effects are additive. - This results in a continuous variation of phenotypes. - Environmental effects cause intervening phenotypes. - e.g., skin color ranges from very dark to very light - e.g., height varies among individual humans
What is cystic fibrosis?
- Chloride ions do not pass normally through a cell membrane, resulting in thick mucus in lungs and other places, often causing infections 21.3 Inheritance of Genetic Disorders 4
What are some X-linked disorders that are recessive? (4)
- Color blindness is most often characterized by red-green color blindness. - Duchenne muscular dystrophy is characterized by wasting of muscles and death by age 21. - Fragile X syndrome is the most common cause of inherited mental impairment. - Hemophilia is characterized by the absence of particular clotting factors; blood clots very slowly or not at all.
What is genotype? What is alleles?
- Genotype: specific genes for a particular trait written with symbols - Alleles: alternate forms of a specific gene at the same position (locus) on a gene (e.g., allele for unattached earlobes and attached lobes); alleles occur in pairs
What is Huntington disease?
- Huntington protein has too many glutamine amino acids, leading to the progressive degeneration of brain cells
What are dominant alleles?
- Masks or suppresses the expression of its complementary allele - Always expressed, even if heterozygous (AA, Aa) - Denoted by a capital letter ex. A for brown hair
What is the law of independent assortment?
- The alleles of different genes are distributed to gametes independently during meiosis - This law applies only if the two genes in question are on different chromosomes
What are recessive alleles?
- Will not be expressed if paired with a dominant allele (heterozygous) - Denoted by a lower case letter ex. a for blond hair - Will only be expressed if individual is homozygous for the recessive allele (aa)
What is multifactorial trait?
- a polygenic trait that is particularly influenced by the environment - e.g., skin color is influenced by sun exposure - e.g., height can be affected by nutrition
What is monohybrid crosses?
- an experimental cross in which parents are identically heterozygous at 1 gene pair -EX: Aa x Aa
What is one-trait cross?
- considers the inheritance of 1 characteristic -EX: WW x Ww
What is two-trait cross?
- considers the inheritance of 2 characteristics -EX: WWTT x WwTT
What is osteogenesis imperfecta?
- defect in collagen synthesis; results in weakened, brittle bones
What is Marfan syndrome?
- defect in the production of the elastic connective tissue protein fibrillin; results in dislocated lens, long limbs and fingers, caved-in chest, and weak wall of aorta
Gametes only carry 1 allele, so if an individual has the genotype Ww, what are the possible gametes that this individual can pass on?
- either a W or a w, but not both
What is phenylketonuria?
- lack of an enzyme needed to make a certain amino acid; affects nervous system development
What is Tay-Sachs disease?
- lack of the enzyme that breaks down lipids in lysosomes resulting in excess buildup of fatty acid proteins in the brain causing eventual blindness, seizures, and paralysis, and death by usually 5 years of age
What is sickle-cell disease?
- red blood cells are sickle-shaped rather than biconcave, resulting in clogged blood vessels
What is phenotype?
- the physical or outward expression of the genotype
What is a dihybrid cross?
-a type of two-trait cross - an experimental cross usually involving parents who are homozygous for different alleles of 2 genes - Results in a 9:3:3:1 genotypic ratio for the offspring
What is Familial hypercholesterolemia?
• 2 mutated alleles lack LDL-cholesterol receptors. • 1 mutated allele has half the normal number of receptors. • 2 normal alleles have the usual number of receptors. • When receptors are completely absent, excessive cholesterol is deposited in various places in the body, including under the skin.
What are some examples of environmental influences on phenotypes? (Himalayan rabbit)
• Himalayan rabbit's coat color is influenced by temperature. • There is an allele responsible for melanin production that appears to be active only at lower temperatures. • The extremities have a lower temperature and thus the ears, nose, paws, and tail are dark in color.
What is preimplantation genetic diagnosis (PGD)?
• If prospective parents carry an allele for a genetic disorder, they may seek assurance that their offspring will be free of the disorder • Following in vitro fertilization (IVF), the zygote divides. • When the embryo has 8 cells, 1 may be removed for genetic testing. • Only embryos that will not have the genetic disorders of interest are placed in the uterus to continue developing.
What is autosomal recessive disorder?
• Individuals must be homozygous recessive to have the disorder • Affected children can have unaffected parents. • Heterozygotes (Aa) have an unaffected phenotype. • Two affected parents will always have affected children. • Affected individuals with homozygous unaffected mates will have unaffected children. • Close relatives who reproduce are more likely to have affected children. • Both males and females are affected with equal frequency.
What is autosomal dominant disorder?
• Individuals that are homozygous dominant and heterozygous will have the disorder • Affected children will usually have an affected parent. • Heterozygotes (Aa) are affected. • Two affected parents can produce an unaffected child. • Two unaffected parents will not have affected children. • Both males and females are affected with equal frequency
What are some autosomal dominant disorders?
• Marfan syndrome • Osteogenesis imperfecta • Huntington disease - Huntington protein has too many glutamine amino acids, leading to the progressive degeneration of brain cells
What is codominance?
• Occurs when the alleles are equally expressed in a heterozygote • Ex: (Type A blood) AA x BB (Type B blood) can lead to AB (Type AB blood that has characteristics of both blood types)
What is Incomplete dominance?
• Occurs when the heterozygote phenotype is intermediate between phenotypes of the 2 homozygotes • EX: (curly hair) CC x SS (straight hair) can lead to CS (wavy hair)
What are linked alleles?
• Physically located on the same chromosome • May be inherited together • May be "shuffled" during crossing over during meiosis
What are some autosomal recessive disorders?
• Tay-Sachs disease • Cystic fibrosis • Phenylketonuria (PKU) • Sickle-cell disease
What is multiple allele inheritance?
• The gene exists in several allelic forms. • A person only has 2 of the possible alleles. • A good example is the ABO blood system. • A and B are codominant alleles. • The O allele is recessive to both A and B; therefore, to have this blood type, you must have 2 recessive alleles.
What are Punnett squares?
• The use of a grid to diagram crosses between individuals by using the possible parental gametes. • These allow one to determine the probability that an offspring will have a particular genotype and phenotype.
What are X-linked disorders?
• These are more often found in males than females because recessive alleles are always expressed. • Most X-linked disorders are recessive • An affected son can have parents who have the normal phenotype. • For a female to have the characteristic, her father must also have it. Her mother must have it or be a carrier. • The characteristic often skips a generation from the grandfather to the grandson. • If a woman has the characteristic, all of her sons will have it.
What is sex-linked inheritance?
• Traits are controlled by genes on the sex chromosomes. • X-linked inheritance - the allele is carried on the X chromosome • Y-linked inheritance - the allele is carried on the Y chromosome • Most sex-linked traits are X-linked.
