HER - Non-Mendelian Inheritance [LESSON]

Mendelian Inheritance

To meet the requirements of Mendelian inheritance, we make the following assumptions:

  • One gene determines one trait.
  • There are only two possible alleles, one dominant and one recessive.
  • Each gene is located on a different autosome pair.

In reality, patterns in heredity can be much more complex. In 1910, Thomas Hunt Morgan started his work with Drosophila melanogaster, a fruit fly. He chose fruit flies because they can be cultured easily, are present in large numbers, have a short generation time, and have only four pair of chromosomes that can be easily identified under the microscope. They have three pair of autosomes and a pair of sex chromosomes. At that time, he already knew that X and Y have to do with the sex of the fly. He used normal flies with red eyes and mutated flies with white eyes and cross bred them. In flies, the wild type eye color is red (XW) and is dominant to white eye color (Xw). He was able to conclude that the gene for eye color was on the X chromosome. This trait was thus determined to be X-linked and was the first X-linked trait to be identified. Males are said to be hemizygous, in that they have only one allele for any X-linked characteristic.

In Drosophila, the gene for eye color is located on the X chromosome.

Sex-linked Inheritance

When a gene is present on the X chromosome, but not on the Y chromosome, it is said to be sex-linked (or X-linked). X-linked genes have different inheritance patterns than genes on non-sex chromosomes (autosomes). That's because these genes are present in different copy numbers in males and females. We can see how sex linkage affects inheritance patterns by considering a cross between two flies, a white-eyed female (XwXw) and a red-eyed male (XWY). If this gene were on a non-sex chromosome, or autosome, we would expect all of the offspring to be red-eyed, because the red allele is dominant to the white allele. What we actually see is that all of the male offspring have the white eye trait because they inherited the white eye allele from mom on the X chromosome and the Y chromosome from dad. All of the females have red eyes.

All male offspring have white eyes and all female offspring have red eyes.

An example of a human sex-linked disorder is hemophilia, which is a blood clotting disorder. Look at the Punnett square below to see the inheritance pattern of hemophilia.

A carrier mother & a father with hemophilia have unaffected daughters and a 50/50 chance in males of having hemophilia.

Watch the video below to learn more about Sex-Linked Inheritance.

Incomplete Dominance

Mendel’s results were groundbreaking partly because they contradicted the (then-popular) idea that parents' traits were permanently blended in their offspring. In some cases, however, the phenotype of a heterozygous organism can actually be a blend between the phenotypes of its homozygous parents. For example, in the snapdragon, Antirrhinum majus, a cross between a homozygous white-flowered plant (CWCW) and a homozygous red-flowered plant (CRCR) will produce offspring with pink flowers (CRCW). This type of relationship between alleles, with a heterozygote phenotype intermediate between the two homozygote phenotypes, is called incomplete dominance.

Cross between homozygous white-flowered plant and homozygous red-flowered plant produce offspring w. pink flowers

Codominance

Closely related to incomplete dominance is codominance, in which both alleles are simultaneously expressed in the heterozygote. For example, the rhododendron flower below displays both phenotypes – the white and red patterns. They did not mix together before expression like in incomplete dominance.

the image shows a red and white patterned flower

Multiple Alleles

Mendel studied just two alleles of his pea genes, but real populations often have multiple alleles of a given gene.  For example, blood type in humans is determined by three alleles, not two. The combinations of alleles we see in human genotypes still just contains two alleles, but there are three to choose from in the population. People with AA or AO have type A blood, people with BB or BO have type B blood, people with AB have type AB blood, and people with OO have type O blood.  You may be able to infer here that O is recessive to both A and B and that A and B are codominant to each other.

Polygenic inheritance

Some characteristics are polygenic, meaning that they’re controlled by a number of different genes. In polygenic inheritance, traits often form a phenotypic spectrum rather than falling into clear-cut categories. Traits such as skin color, hair color, height, and eye color in humans are all polygenic.

Pleiotropy

A single gene controls expression in multiple features

A human genetic disorder called Marfan syndrome is caused by a mutation in one gene, yet it affects many aspects of growth and development, including height, vision, and heart function. This is an example of pleiotropy, or one gene affecting multiple characteristics.

Pedigrees

Pedigrees are used to analyze the pattern of inheritance of a particular trait throughout a family. Pedigrees show the presence or absence of a trait as it relates to the relationship among parents, offspring, and siblings.

Key showing female as a circle, male as a square, shaded box as affected individual, and other symbols for rare events

By analyzing a pedigree, we can determine genotypes, identify phenotypes, and predict how a trait will be passed on in the future. The information from a pedigree makes it possible to determine how certain alleles are inherited: whether they are dominant, recessive, autosomal, or sex-linked.

To start reading a pedigree:

  1. Determine if the chart shows an autosomal or sex-linked (usually X-linked) trait. For example, in X-linked recessive traits, males are much more commonly affected than females. In autosomal traits, both males and females are equally likely to be affected (usually in equal proportions).
  2. Determine whether the trait is dominant or recessive. If the trait is dominant, one of the parents must have the trait. Dominant traits will not skip a generation. If the trait is recessive, neither parent is required to have the trait since they can be heterozygous.  Sex-linked traits can be considered recessive.
What conclusions can you draw if many family members have the trait?
  1. It is a common misconception to think that dominant traits show up more in the population. A good example of this is blood type. Type O blood is recessive, yet it is more common in the population!

Try the Pedigrees learning activity below. Click the Show Answer button to reveal the answer, then click the arrow at the bottom of the activity.

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