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 generalappearance is referred to as the phenotype.

For each organism, the geneticmakeup of the alleles is referred to as the genotype.

Patterns of inheritance

Self fertilization of the F1 generation will result in a second generation where¾ 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 beable to mathematically predict the outcome of various crosses.

Patterns of Mendelian inheritance – a closer look

In the parental generation, heterozygousindividuals produce gametes all with the same allele type.

The offspring generation howeveris heterozygous and will thereforeproduce gametes that have differentalleles.

The ratios of the alleles in the gametesare predictable; in this case 50:50

(note that the dominant alleles is typicallymarked with a capital letter symbol, and the recessive allele is marked with a lowercase letter symbol)

Patterns of Mendelian inheritance – a closer look

The pattern by which the gametesfertilize each other will determinethe phenotype of the next generation.

If our sample sizes are large enough,then we can actually mathematicallypredict 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 heterozygousplant, then we know that ½ of the eggs and ½ of the sperm will have the dominant (P) allele, and thatthe 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 dominantVersus recessive genes to predict thePhenotypic 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 thegenotype.

However, since the patterns of inheritanceare predictable, we can use simple experimentationand Punnett squares to establish genotypes.

Patterns of inheritance

Mendel went beyond consideringjust 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 foundon different chromosomes, then theirdistribution will be independent of eachother. In these cases, Punnett squareanalysis can still be a powerful tool inpredicting and studying patterns ofinheritance.

At this point, you should be able to pickany two genotypes (consider the possibilitiesfrom the chart to the right), cross them,and be able to predict the genotypic as well asphenotypic 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 traitsthat 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

In some cases however, linked genes can be separated by recombinationdue to crossing over:

Patterns of inheritance for linked genes typically follow the patterns of a single gene.

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These recombinations will cause variation on the basic patterns of inheritancepredicted by Mendelian genetics and Punnett squares.

Patterns of inheritance – Sex determination in mammals

In mammals, XX offspring are femalewhereas XY offspring are male. This patternMay be different for other organisms…

The Y chromosome typically carries just afew genes, whereas the X chromosome maycarry many genes that may or may not haveanything to do with the sexual outcome of the animal.

Genes that are on one sex chromosome butnot 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 havephenotypes 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 overOthers, 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 beregulatory 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 observedin humans – we will consider some of these themes in the laboratory section of this lecture.