Illustration to show the inheritance of dominant and recessive alleles for eye colour. Image credit: Genome Research Limited. Genes are small sections of DNA within the genome that code for proteins. They contain the instructions for our individual characteristics — like eye and hair colour. Inheritance is the process by which genetic information is passed on from parent to child. This is why members of the same family tend to have similar characteristics.
Genetic variation is a term used to describe the variation in the DNA sequence in each of our genomes. Genetic variation is what makes us all unique, whether in terms of hair colour, skin colour or even the shape of our faces. Haemophilia A and B are two disorders characterised by slow and inefficient formation of blood clots leading to prolonged bleeding and spontaneous internal bleeding.
Single gene disorders are caused by DNA changes in one particular gene, and often have predictable inheritance patterns. Thus, the multiple alleles at the Lf locus represent an allelic series, with each allele being dominant over the next allele in the series.
Mendel's early work with pea plants provided the foundational knowledge for genetics, but Mendel's simple example of two alleles, one dominant and one recessive, for a given gene is a rarity.
In fact, dominance and recessiveness are not actually allelic properties. Rather, they are effects that can only be measured in relation to the effects of other alleles at the same locus. Furthermore, dominance may change according to the level of organization of the phenotype.
Variations of dominance highlight the complexity of understanding genetic influences on phenotypes. Murfet, I. Flowering in Pisum : Multiple alleles at the Lf locus.
Heredity 35 , 85—98 Parsons, P. The evolution of overdominance: Natural selection and heterozygote advantage. Nature , 7—12 link to article. Stratton, F. The human blood groups. Nature , link to article. Chromosome Theory and the Castle and Morgan Debate.
Discovery and Types of Genetic Linkage. Genetics and Statistical Analysis. Thomas Hunt Morgan and Sex Linkage. Developing the Chromosome Theory. Genetic Recombination. Gregor Mendel and the Principles of Inheritance. Mitosis, Meiosis, and Inheritance. Multifactorial Inheritance and Genetic Disease. Non-nuclear Genes and Their Inheritance. Polygenic Inheritance and Gene Mapping. Sex Chromosomes and Sex Determination. Sex Determination in Honeybees. Test Crosses.
Biological Complexity and Integrative Levels of Organization. Genetics of Dog Breeding. Human Evolutionary Tree. Mendelian Ratios and Lethal Genes. Environmental Influences on Gene Expression. Epistasis: Gene Interaction and Phenotype Effects. Genetic Dominance: Genotype-Phenotype Relationships. Looking at this, you might conclude that the dominant phenotype is twice as common as the recessive one.
But you would probably be wrong. Recessive alleles can be present in a population at very high frequency. Consider eye color. Eye color is influenced mainly by two genes, with smaller contributions from several others. People with light eyes tend to carry recessive alleles of the major genes; people with dark eyes tend to carry dominant alleles. In Scandinavia, most people have light eyes—the recessive alleles of these genes are much more common here than the dominant ones.
Mode of inheritance has nothing to do with whether an allele benefits an individual or not. Take rock pocket mice, where fur color is controlled mainly by a single gene. The gene codes for a protein that makes dark pigment. Some rock pocket mice have dark fur, and some have light fur. The dark-fur allele is dominant, and the light-fur allele is recessive. But not all diseases alleles are recessive. Keratin proteins link together to form strong fibers that strengthen hair, fingernails, skin, and other tissues throughout the body.
There are several genetic disorders involving defects in keratin genes, and most of them have dominant inheritance patterns. To see how defective keratin genes can lead to a genetic disorder, see Pachyonychia Congenita. What are Dominant and Recessive?
The terms are confusing and often misleading Dominant and recessive inheritance are useful concepts when it comes to predicting the probability of an individual inheriting certain phenotypes, especially genetic disorders. Breeding the flies shown in this Punnett square will determine the distribution of phenotypes among their offspring. If the female parent has the genotype BB, all of the offspring will have brown bodies Figure 9, Outcome 1.
In this way, the genotype of the unknown parent can be inferred. Figure 9. Figure The phenotypic ratio is brown body: black body.
This observation forms the second principle of inheritance, the principle of segregation, which states that the two alleles for each gene are physically segregated when they are packaged into gametes, and each parent randomly contributes one allele for each gene to its offspring. Can two different genes be examined at the same time? Figure The possible genotypes for each of the four phenotypes. The dihybrid cross: charting two different traits in a single breeding experiment.
Figure These are all of the possible genotypes and phenotypes that can result from a dihybrid cross between two BbEe parents. On the upper left, the female parent genotype is uppercase B lowercase b, uppercase E lowercase e. Uppercase B, uppercase E is labeled to the left of the top quadrant; lowercase b, lowercase e is labeled outside the second left quadrant; uppercase B, lowercase e is labeled outside the third left quadrant; and lowercase b, uppercase E is labeled outside the fourth left quadrant.
On the upper right, the male parent genotype is also uppercase B lowercase b, uppercase E lowercase e. Uppercase B, uppercase E is labeled to the right of the top quadrant; lowercase b, lowercase e is labeled to the outside the second right quadrant; uppercase B, lowercase e is labeled outside the third right quadrant, and lowercase b, uppercase E is labeled outside the fourth right quadrant. The offsprings' genotype and phenotype is represented in each of the cells of the Punnett square.
Nine of the 16 cells contain brown-bodied flies with red eyes. Of these nine flies, one has the genotype uppercase B, uppercase B, uppercase E uppercase E; four have the genotype uppercase B lowercase b, uppercase E lowercase e; two have the genotype uppercase B uppercase B, uppercase E lowercase e; and two have the genotype uppercase B lowercase b, uppercase E uppercase E.
Three cells contain brown-bodied flies with brown eyes. Of these three flies, one has the genotype uppercase B uppercase B, lowercase e lowercase e and two have the genotype uppercase B lowercase b, lowercase e lowercase e.
Three cells contain black-bodied flies with red eyes. Of these three flies, one has the genotype lowercase b lowercase b, uppercase E uppercase E and two have the genotype lowercase b lowercase b, uppercase E lowercase e. The final cell contains a black-bodied fly with brown eyes; this fly has the genotype lowercase b lowercase b, lowercase e, lowercase e.
The impact of Mendel's principles. Seminal experiments on inheritance. Key Questions What is non-nuclear inheritance? How can the same genotype give you a different disease? Who was Gregor Mendel? Key Concepts testcross dihybrid cross Principle of Independent Assortment. Topic rooms within Genetics Close.
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