Patterns of inheritance I. Observation: Offspring resemble their parents II. Conclusion: Offspring inherit their physical characteristics from their parents III. The patterns of inheritance are not always obvious A. Sometimes, traits in offspring appear to be a blend of the traits in the parents B. Other times, the traits in the offspring appear to be like one or the other parent C. Yet other times, the traits seem nothing like the parents. Some traits may have “skipped” one or more generations. IV. So what is going on?

Patterns of inheritance Terminology: Genes: Segments of the DNA on chromosomes that code for a specific protein Locus (loci): The specific physical location of a gene on the chromosome Homologous chromosomes: Chromosomes that carry the same genes. Since most cells are diploid, they have a set of two chromosomes and therefore two copies of each gene. Alleles: Although homologous chromosomes carry the same “type” of genes, the specific sequence of the same gene on the two homologous chromosomes may be slightly different. The different forms of the same gene found on the different chromosomes are referred to as alleles.

Patterns of inheritance (Mendel’s experiments explained) Parental generation are true breeding: they are homozygous, which means that they have the exact same gene (i.e. allele) form for color on their homologous chromosomes. The first generation looks like one of the parents. Genetically however, it is heterozygous, which means it has two different allele forms, one from each parent. The first generation flowers look purple, because the purple allele is dominant, And the white allele is recessive. Terminology: The purple parent is referred to as homozygous dominant. The white parent is referred to as homozygous recessive. The purple offspring is referred to as heterozygous.

Patterns of inheritance Terminology (cont.): For each organism, the general appearance is referred to as the phenotype. For each organism, the genetic makeup of the alleles is referred to as the genotype.

Patterns of inheritance Self fertilization of the F1 generation will result in a second generation where ¾ of the offspring are purple and ¼ of the offspring are white. Mendelian genetics will allow us to explain what happens here, and we will be able to mathematically predict the outcome of various crosses.

Patterns of Mendelian inheritance – a closer look In the parental generation, heterozygous individuals produce gametes all with the same allele type. (note that the dominant alleles is typically marked with a capital letter symbol, and the recessive allele is marked with a lower case letter symbol) The offspring generation however is heterozygous and will therefore produce gametes that have different alleles. The ratios of the alleles in the gametes are predictable; in this case 50:50

Patterns of Mendelian inheritance – a closer look The pattern by which the gametes fertilize each other will determine the phenotype of the next generation. If our sample sizes are large enough, then we can actually mathematically predict both the genotypic and the phenotypic outcome of the next generation.

Patterns of inheritance – The Punnett Square If we know that we are self fertilizing a heterozygous plant, then we know that ½ of the eggs and ½ of the sperm will have the dominant (P) allele, and that the other half will have the recessive (p) allele. Next, since we know that during fertilization, the genetic material of the egg and the sperm combine, then we can simply cross multiply our alleles to come up with the genotype of the next generation; in this case: ¼ PP : ½ Pp : ¼ pp Now that we have predicted the genotype, We can use our knowledge of dominant Versus recessive genes to predict the Phenotypic ratio of our offspring; in this case: ¾ purple : ¼ white

Patterns of inheritance – The Punnett Square By simply looking at the pea plant flowers, we may or may not be able to predict the genotype. However, since the patterns of inheritance are predictable, we can use simple experimentation and Punnett squares to establish genotypes.

Patterns of inheritance Mendel went beyond considering just one trait though. He considered a number of traits and how combinations of these traits are inherited.

Patterns of inheritance If the alleles being studied are found on different chromosomes, then their distribution will be independent of each other. In these cases, Punnett square analysis can still be a powerful tool in predicting and studying patterns of inheritance. At this point, you should be able to pick any two genotypes (consider the possibilities from the chart to the right), cross them, and be able to predict the genotypic as well as phenotypic outcome of the next generation.

Patterns of inheritance For multiple genes, the patterns of inheritance are predictable because of the independent (or random) assortment of alleles

Patterns of inheritance Punnet squares can be used to study inheritance pattern of multiple traits that are not on the same chromosome – i.e. are independently inherited. Genes that are located on the same chromosomes however do now follow the same pattern because they are inherited together and are said to be “linked”. Patterns of inheritance for linked genes typically follow the patterns of a single gene.

Patterns of inheritance Patterns of inheritance for linked genes typically follow the patterns of a single gene. In some cases however, linked genes can be separated by recombination due to crossing over: >>>> These recombinations will cause variation on the basic patterns of inheritance predicted by Mendelian genetics and Punnett squares.

Patterns of inheritance – Sex determination in mammals In mammals, XX offspring are female whereas XY offspring are male. This pattern May be different for other organisms… The Y chromosome typically carries just a few genes, whereas the X chromosome may carry many genes that may or may not have anything to do with the sexual outcome of the animal. Genes that are on one sex chromosome but not on the other are said to be sex-linked. What do you think would be the pattern of inheritance in sex-linked genes?

Variations on the Mendelian Theme 1. Alleles may display incomplete dominance – heterozygous individuals may have phenotypes that are intermediates between phenotypes of the homozygotes.

Variations on the Mendelian Theme (cont.) 2. A single gene may have multiple alleles, some of which may be dominant over Others, whereas other alleles may be codominant (e.g. human blood groups)

Variations on the Mendelian Theme (cont.) 3. Some traits may be influenced by several genes – polygenic inheritance (e.g. human eye or skin color)

Variations on the Mendelian Theme (cont.) 4. Single genes may also have multiple effects on phenotype – they may be regulatory proteins that affect other genes. 5. The environment can influence the expression of some genes.

Patterns of Mendelian inheritance in humans Mendelian inheritance can be seen in many of the phenotypic characteristics observed in humans – we will consider some of these themes in the laboratory section of this lecture.