Introduction to Mendelian Genetics

Notes: Patterns of Inheritance: What Is The Physical Basis of Inheritance? Gene = A unit of DNA that encodes information needed to produce proteins, cells, and entire organisms - Can be a few hundred to many thousands of nucleotides in length - The location of a gene on a chromosome is called its locus - In the ordinary body cells (not sperm or eggs) of many organisms the chromosomes occur in pairs. - Each species has a characteristic number of chromosomes - Human cells contain 46 chromosomes, 23 pairs Chromosomes: Humans = Autosomes (22 pairs) + 1 pair of sex chromosomes - Genes are sequences of nucleotides at specific locations on chromosomes - Homologous chromosomes carry the same kinds of genes for the same characteristics - Genes for the same characteristic are found at the same loci on both homologous chromosome Homologous chromosomes (homologues) = A pair of chromosomes that contain similar genes. - Genes for a characteristic found on homologous chromosomes may not be identical - Alternative versions of genes found at the same gene locus are called alleles. - Alleles are formed by mutations. Alleles: - An organism’s two alleles may be the same or different - Each cell carries two alleles per characteristic, one on each of the two homologous chromosomes - If both homologous chromosomes carry the same allele (gene form) at a given gene locus, the organism is homozygous at that locus - If two homologous chromosomes carry different alleles at a given locus, the organism is heterozygous at that locus, also called hybrid The Punnett Square: - The Punnett square method predicts offspring genotypes and phenotypes from combinations of parental gametes 1. Assign letters to the different alleles of the characteristic under consideration (uppercase for dominant, lowercase for recessive) 2. Determine the possible gametes from both parents 3. Write the gametes from each parent, along each side of a 2 x 2 grid (Punnett square) 4. Fill in the genotypes of each pair of combined gametes in the grid. 5. Determine the genotypic ratio based on the outcome of the Punnett Square. 6. Based on which traits are dominant, and which are recessive, calculate the predicted phenotypic ratio. How Were The Principles of Inheritance Discovered? Gregor Mendel - Austrian monk in a monastery in the late 1800s - Discovered patterns of inheritance and many essential facts about genes, alleles, and the distribution of alleles in gametes during sexual reproduction - He chose edible pea plant for his experiments - Allowed him to see patterns in the way plant characteristics were inherited - Pea flowers have stamens, the male structures that produce pollen. Pollen contains the sperm (male gametes); sperm are gametes and pollen is the vehicle - Pea flowers have carpels, female structures housing the ovaries, which produce the eggs (female gametes) - Pea flower petals enclose both male and female flower parts and prevent entry of pollen from another pea plant Flowers of the Edible Pea Plants: - Because of their structure, pea flowers naturally self-fertilize - Pollen from the stamen of a plant transfers to the carpel of the same plant, where the sperm then fertilizes the plant’s eggs The Key To Mendel’s Success: - He chose a simple experimental design - He chose to study individual characteristics (called traits) that had unmistakably different forms, such as white versus purple flowers - He started out by studying only one trait at a time - He followed the inheritance of these traits for several generations, counting the numbers of offspring with each type of trait - By analyzing these numbers, he saw the basic patterns of inheritance emerge How Are Single Traits Inherited? - Pea plants that are homozygous for a particular characteristic always produce the same phenotype - If a plant is homozygous for purple flowers, it will always produce offspring with purple flowers - Plants that are homozygous for a characteristic are described as true-breeding - A genetic cross is the mating of pollen and eggs (from same or different parents) - The parents used in a cross are part of the parental generation (known as P) - The offspring of the P generation are members of the first filial generation (F1 ) - Offspring of the F1 generation are members of the F2 generation - Mendel was able to mate two different plants by (cross-fertilization) - Female parts (carpels) were dusted with pollen from other selected plants Mendel Flower Color Experiments: - Crossed a true-breeding (homozygous) purple flower plant with a true-breeding (homozygous) white-flower plant (the P generation) - The F1 generation consisted of all purple flowered plants - What had happened to the white-flowered trait? - Mendel allowed the F1 generation to self-fertilize - The F2 was composed of 3/4 purple-flowered plants and 1/4 white-flowered plants, a ratio of 3:1 - The results showed that the white trait had not disappeared in the F1 but merely was hidden - Mendel then allowed the F2 generation to self fertilize - In the F3 generation, all the white-flowered F2 plants produced white-flowered offspring - These proved to be true-breeding (homozygous) - In the F3 generation, self-fertilized purpleflowered F2 plants produced 3 offspring –1 out of the 3 were true-breeding for purple –The other 2 were heterozygous, and produced both purple- and white-flowered offspring. - So, Mendel was able to determine that out of the 4 offspring in the F2 generation, 1 was homozygous for purple, 1 was homozygous for white, and the other 2 were heterozygous - The inheritance of dominant and recessive alleles on homologous chromosomes can explain the results of Mendel’s crosses Five-part hypothesis explains the inheritance of single traits 1. Each trait is determined by pairs of genes; each organism has two alleles for each gene, one on each homologous chromosome - True-breeding white-flowered plants have different alleles than true-breeding purple-flowered plants 2. When two different alleles are present in an organism, the dominant allele may mask the recessive allele, even though the recessive allele is still present In edible peas the purple-flower trait is dominant to the white-flower trait 3. The pairs of alleles on homologous chromosomes separate, or segregate, from each other during meiosis, which is known as Mendel’s law of segregation 4. Chance determines which allele is included in a given gamete—because homologous chromosomes separate at random during meiosis; the distribution of alleles to the gametes is also random 5. True-breeding organisms have two copies of the same allele for a given gene and are homozygous for that gene; hybrid organisms have two different alleles for a given gene and are heterozygous for that gene The hypothesis explains Mendel’s results with peas - The particular combination of the two alleles carried by an individual is called the genotype - The physical expression of the genotype is known as the phenotype (for example, purple or white flowers - There are two alleles for a given gene characteristic (such as flower color) ● Let P stand for the dominant purple-flowered allele: A homozygous purple-colored plant has two alleles for purple flower color (PP) and produces only P gametes ● Let p stand for the recessive white-flowered allele: A homozygous white-colored plant has two alleles for white flower color (pp) and produces only p gametes ● A cross between a purple-flowered plant (PP) and a white-flowered plant (pp) produces all purple-flowered F1 offspring, with a Pp genotype ● Dominant P gametes from purple-flowered plants combined with recessive p gametes from white-flowered plants to produce hybrid purple-flowered plants (Pp) - A test cross is used to deduce whether an organism with a dominant phenotype is homozygous or heterozygous 1. Cross the unknown dominant-phenotype organism (P_) with a homozygous recessive organism (pp) 2. If the dominant-phenotype organism is homozygous dominant (PP), only dominant-phenotype offspring will be produced (Pp) 3. If the dominant-phenotype organism is heterozygous (Pp), approximately half the offspring will be of recessive phenotype (pp) How Is Sex Determined In The Offspring? Mammals have a set of sex chromosomes that dictate gender: - Females have two X chromosomes. Males have an X chromosome and a Y chromosome - The rest of the (non-sex) chromosomes occur in identical pairs and are called autosomes - For organisms in which males are XY and females are XX, the sex chromosome carried by the sperm determines the sex of the offspring - During sperm formation, each sperm receives either the X or the Y chromosome, along with a copy of each of the autosomes - Because the female has only X sex chromosomes, the unfertilized egg will always have just an X chromosome - If the egg is fertilized by a sperm with a Y chromosome, a male results; if fertilized by an Xbearing sperm, a female is produced - Many traits do not follow simple Mendelian rules of inheritance - Not all traits are completely controlled by a single gene - A trait may not be completely dominant to another Incomplete Dominance: - In the genes studied by Mendel, one allele was dominant over the other, which was recessive - Some alleles, however, are incompletely dominant over others When the heterozygous phenotype is intermediate between the two homozygous phenotypes, the pattern of inheritance is called incomplete dominance Incomplete Dominance: - Human hair texture is influenced by a gene with two incompletely dominant alleles, C1 and C2 - A person with two copies of the C1 allele has curly hair - Someone with two copies of the C2 allele has straight hair - Heterozygotes (with the C1, C2 genotype) have wavy hair - If two wavy-haired people mate, their children could have any of the three hair types: curly (C1, C1), wavy (C1, C2 ), or straight (C2, C2) - A single gene may have multiple alleles - An individual may have at most two different gene alleles - A species may have multiple alleles for a given characteristic - However, each individual still carries two alleles for this characteristic Human Blood Types: Human blood group genes produce blood types A, B, AB, and O - There are three alleles in this system: A, B, and o - Alleles A and B code for enzymes that add different sugar molecules to the ends of glycoproteins that protrude from red blood cells - Allele o codes for a nonfunctional enzyme that doesn’t add any sugar molecules - Blood types A, B, AB, and O arise as a result of the actions of these alleles - Alleles A and B are dominant to o - People with AA or Ao genotypes have blood type A; people with BB or Bo genotypes have blood type B; people with oo genotypes have blood type O - AB individuals have both the A and the B allele, so they produce both types of enzymes - Consequently, the plasma membranes of their red blood cells have both A and B glycoproteins When heterozygotes express the phenotypes of both of the homozygotes (in this case, both A and B glycoproteins), the pattern of inheritance is called codominance ● People make antibodies to the type of glycoproteins they lack ● People with type A blood make B antibodies; people with type B blood make A antibodies ● People with type O blood make both type A and type B antibodies; type AB blood groups make no antibodies - The antibodies cause red blood cells that bear foreign glycoproteins to clump together and rupture - This is why blood type must be determined and matched carefully before a blood transfusion is made Environment + Gene Expression; The environment influences the expression of genes: - The environment in which an organism lives profoundly affects its phenotype - Newborn Siamese cats demonstrate the effect of environment on phenotype - A Siamese cat has the genotype for dark fur all over its body - However, the enzyme that produces the dark pigment is inactive at temperatures above 93°F (34°C) - When kittens are in the all-encompassing warmth of their mother’s uterus, the enzyme is inactive and they are born with pale fur everywhere on their bodies - After birth, the ears, nose, paws, and tail become cooler than the rest of the body, and dark pigment is produced there in the pattern characteristic of the breed Sex + Human Genetic Disorders: Some human genetic disorders are sex-linked - The X chromosome contains many genes that have no counterpart on the Y chromosome - Because males have only one X chromosome, they have no other allele to exert dominance over a sex-linked (X-linked) allele causing disease - Consequently, sex-linked diseases, such as hemophilia tend to occur more often in males The incorrect separation of chromosomes in meiosis is known as nondisjunction - Nondisjunction causes gametes to have too many and too few chromosomes - Most embryos that arise from fusion of gametes with abnormal chromosome numbers spontaneously abort, but some survive to birth and beyond Autosomes + Human Genetic Disorders: Some genetic disorders are caused by abnormal numbers of autosomes: - In trisomy 21 (Down syndrome), afflicted individuals have three copies of chromosome 21 - Down syndrome includes several distinctive physical characteristics, including weak muscle tone, a small mouth held partially open because it cannot accommodate the tongue, and distinctively shaped eyelids - Down syndrome is also characterized by low resistance to infectious diseases, heart malformations, and varying degrees of mental retardation, often severe