Mendelian Inheritance Patterns

• Chapter 11 focuses on Mendelian Patterns of Inheritance

Gregor Mendel

• The blending concept of inheritance suggests parents with contrasting appearances produce offspring with intermediate appearances.
• Mendel formulated the particulate theory of inheritance.
• Mendel proposed the laws of segregation and independent assortment.
• Inheritance involves the reshuffling of genes from generation to generation.
• Mendel was an Austrian monk, who studied science and mathematics at the University of Vienna.
• He conducted breeding experiments on the garden pea Pisum sativum.
• His experiments on the inheritance of simple traits disproved the blending hypothesis.
• Garden peas are easy to cultivate, have a short generation time, and are normally self-pollinating but can be cross-pollinated by hand.
• True-breeding varieties are available, and they have simple, objective traits.
• Pollen is transferred from the anther of one plant to the stigma of another through cross-pollination.

Mendel's Laws

• Mendel performed cross-pollination experiments using true-breeding (homozygous) plants.
• He chose varieties that differed in only one trait (monohybrid cross) and performed reciprocal crosses.
• Parental generation is P, first filial generation offspring is , and second filial generation offspring is .
• He formulated the law of segregation.
• Each individual has a pair of factors (alleles) for each trait.
• The factors (alleles) segregate (separate) during gamete (sperm and egg) formation.
• Each gamete contains only one factor (allele) from each pair of factors.
• Additionally, fertilization gives the offspring two factors for each trait.
• Each trait in a pea plant is controlled by two alleles (alternate forms of a gene).
• The dominant allele (capital letter) masks the expression of the recessive allele (lowercase).
• Alleles occur on a homologous pair of chromosomes at a particular gene locus.
• Homozygous means identical alleles.
• Heterozygous means different alleles.
• Genotype refers to the two alleles an individual has for a specific trait.
• A genotype can either be homozygous or heterozygous.
• Phenotype refers to the physical appearance of the individual.
• Dominant and recessive alleles represent DNA sequences that code for proteins.
• The dominant allele codes for the protein associated with the normal gene function within the cell.
• The recessive allele represents a "loss of function".
• During meiosis I, homologous chromosomes separate, causing the two alleles to separate from each other.
• Meiosis explains Mendel's law of segregation and why only one allele for each trait is in a gamete.
• In a dihybrid cross true-breeding plants differing in two traits are used.
• Mendel tracked each trait through two generations, starting with true-breeding plants differing in two traits.
• The plants showed both dominant characteristics, plants self-pollinated, and phenotypes were observed in the plants.
• Mendel formulated the law of independent assortment.
• The pair of factors for one trait segregate independently of the factors for other traits with all possible combinations of factors occurring in the gametes.
• Mendel's laws of probability allow calculations to predict genotypes and phenotypes among offspring, including using a Punnett square which shows a 50% chance.
• Testcrosses use individuals with a recessive phenotype who always have the homozygous recessive genotype.
• Individuals with a dominant phenotype have an indeterminate genotype.
• To determine the genotype of an individual, cross the dominant phenotype with a homozygous recessive individual.

Mendelian Patterns of Inheritance and Human Disease

• Genetic disorders are medical conditions caused by alleles inherited from parents.
• An autosome is any chromosome other than a sex chromosome (X or Y).
• Genetic disorders caused by genes on autosomes are called autosomal disorders.
• For some genetic disorders which are autosomal dominant, an individual with AA or Aa has the disorder, while an individual with aa does NOT have the disorder.
• For some genetic disorders which are autosomal recessive, an individual with AA or Aa does NOT have the disorder.
• An individual with the autosomal recessive phenotype can be a carrier of the disorder.
• An individual with the aa autosomal recessive DOSE have the disorder.
• Autosomal recessive disorders, most affected children have unaffected parents, heterozygotes (Aa) have an unaffected phenotype.
• Two affected parents will always have affected children, and close relatives who reproduce are more likely to have affected children.
• Both males and females are affected with equal frequency.
• If both parents carry one copy of a recessive gene, they are unaffected but can have a child with two copies of the gene who is affected.
• Methemoglobinemia is a relatively harmless disorder where accumulation of methemoglobin in the blood causes skin to appear bluish-purple.
• In Cystic Fibrosis, mucus in bronchial tubes and pancreatic ducts is particularly thick and viscous.
• Two parents with a dominantly inherited disorder will be affected by one copy of the gene
• In Osteogenesis Imperfecta, it is characterized by weakened, brittle bones.
• Most cases of Osteogenesis Imperfecta are caused by mutation in genes required for the synthesis of type I collagen.
• Huntington Disease is a neurological disease that leads to progressive degeneration of brain cells and is caused by a mutated copy of the gene for a protein called huntingtin.
• Hereditary Spherocytosis is caused by a mutation in the ankyrin-1 gene, causing red blood cells to become spherical, fragile, and burst easily.
• In Autosomal dominant disorders, affected children have an affected parent, heterozygotes (Aa) are affected, and two affected parents can produce an unaffected child.
• Additionally, two unaffected parents will not have affected children, and both males and females are affected with equal frequency.

Beyond Mendelian Inheritance

• Some traits are controlled by multiple alleles where the gene exists in several allelic forms with each individual having two alleles.
• For ABO blood types there is antigen on red blood cells, anti-B antibody in plasma and antigen on red blood cells and anti-A antibody in plasma.
• Also i = Neither A nor B antigens on red blood cells, both anti-A and anti-B antibodies in plasma.
• The ABO blood type is an example of codominance as well.
• More than one allele is fully expressed and both alleles expressed in the presence of the other.
• Incomplete dominance is where a heterozygote has a phenotype intermediate between that of either homozygote.
• Homozygous red has a red phenotype.
• Homozygous white has a white phenotype.
• Heterozygote has a pink (intermediate) phenotype with phenotype revealing the genotype without a testcross.
• In Familial Hypercholesterolemia (FH), homozygotes for the mutant allele develop fatty deposits in the skin and tendons and may have heart attacks during childhood.
• Also heterozygotes may suffer heart attacks during early adulthood
• Homozygotes for the normal allele do not have any issues.
• Pleiotropy occurs when a single mutant gene affects two or more distinct and seemingly unrelated traits.
• Marfan syndrome is linked to a mutated gene FBN1 on chromosome 15 which codes for the fibrillin protein.
• Marfan syndrome results in disproportionately long arms, legs, hands, and feet.
• A weakened aorta and poor eyesight are also results of Marfan syndrome.
• Polygenic inheritance occurs when a trait is governed by two or more sets of alleles, where each dominant allele has a quantitative effect on the phenotype.
• These effects are additive.
• Polygenic inheritance also results in continuous variation of phenotypes within a population with traits that may also be affected by the environment.
• Examples of polygenic inheritance are human skin color, height, and eye color.
• Multifactorial traits are controlled by polygenes and subject to environmental influences.
• Examples of multifactorial traits are temperature, human skin, and height.
• For Himalayan rabbits, the enzyme encoded by the gene involved in producing melanin is active only at low temperature.
• Black fur occurs at the extremity, where heat is lost.
• The term X-linked is used for genes that have nothing to do with gender.
• X-linked genes are carried on the X chromosome and the Y chromosome does not carry these genes.
• Most sex-linked experiments are performed on fruit flies, because they can be easily and inexpensively raised in simple laboratory glassware.
• Fruit flies have a similar sex chromosome pattern to humans, therefore experiment results apply to humans.
• Color blindness, Muscular dystrophy, and Hemophilia are examples of human X-Linked disorders.
• X-linked Recessive Disorders affect more males than females.
• An affected son can have parents who have the normal phenotype.
• For a female to have the characteristic, her father must also have it and 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.