What Does Heterozygous Mean?

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In heterozygous inheritance, the genes come from two parent cells to reproduce, and it is present in animals, humans and plants. There arer several instances of heterozygous genes including complete dominance, co-dominance and heterozygous mutations.

What Are Heterozygous Genes?

In all diploid organisms that contain two sets of chromosomes, the term heterozygous means that an individual formed from two parent cells has two different alleles for a single specific trait. Chromosomes contain alleles as a specific DNA trait or gene. You inherit alleles from both parents, in the case of humans, with half from your mother and half from your father.

This is the same process in animals and plants too. The cells contain sets of two homologous chromosomes, meaning that the sets appear in the same position for the same trait on each pair of chromosomes. Homologous chromosomes have the same genetic makeup, but the alleles can differ to determine which traits are expressed in a cell.

What Is a Heterozygous Trait?

A heterozygous trait is when there are two sets of chromosomes in the same area, as alleles are different from each other. One signifies the trait from the mother and one from the father cell, but the two are not the same. For example, if a mother has brown hair, and a father has blond hair, the dominant trait of one of the parents will control the trait or hair color of the child.

What Are Dominant and Recessive Traits?

When two alleles are different on their respective chromosomes from each parent, they may have dominant or recessive genes or traits. The dominant trait is the one that you can see or notice, such as an outward appearance, or it can be a trait that causes a habit, such as biting your nails. The heterozygous recessive trait in this case is masked by the heterozygous dominant trait, so it will not be observed as the dominant trait. In a case where the dominant fully masks the recessive trait, it is called complete dominance.

What Is Incomplete Dominance?

In the case of incomplete dominance, one heterozygous allele is dominant, and one is recessive, however, the dominant trait only partially masks the recessive trait. Instead, a different phenotype is created that is a combination of both of the alleles' phenotypes. For example, if one human parent has a dark skin tone and dark hair, and the other has very light skin and blonde hair, a case of incomplete dominance would be when a child has a medium skin tone, which is a mixture of both of the parental traits.

What Is Co-Dominance?

In the case of co-dominance in genetics, both of the heterozygous alleles are expressed fully in the phenotype from both parents. This can be seen by examining blood types of offspring. If one parent has the blood type of A, and the other parent has blood type B, the child will have a co-dominance blood type of AB. In this case, each of the two different blood types is fully expressed and equally expressed to be co-dominant.

What Is Homozygous?

Homozygous is in essence the opposite of heterozygous. A person with a homozygous trait has alleles that are very similar to each other. Homozygotes only produce homozygous offspring. The offspring may be homozygous dominant expressed as RR or they may be homozygous recessive expressed as rr for a trait.

Homozygous individuals may not have both recessive and dominant traits expressed as Rr. Both heterozygous and homozygous offspring can be born to a heterozygote. The offspring in this case can have dominant and recessive alleles that are expressed in complete dominance, incomplete dominance or even co-dominance.

What Is a Dihybrid Cross in Genetics?

A dihybrid cross is made when the two parent organisms differ in their two traits. The parent organisms have different pairs of alleles for each trait. One parent has homozygous dominant alleles, and the other has the opposite as seen in heterozygous recessive. This makes each parent a complete opposite from the other. The offspring that is produced by the two parent organisms are all heterozygous for all of the specific traits. All the offspring have a hybrid genotype and express the dominant phenotypes for each and every trait.

For example, examine a dihybrid cross in seeds where the two traits being studied are the seed shape and color. One plant is homozygous for both dominant traits of shape and color represented as (YY) for yellow seed color and (RR) for seed shape of round. The genotype is (YYRR). The other plant is the opposite and has homozygous recessive traits as green in seed color and wrinkles in seed shape expressed as (yyrr). When these two plants are crossbred, the result is all heterozygous for yellow as the seed color and round as the seed shape or (YrRr). This is true for the first offspring or F1 generation of all the hybrid cross plants from the same two parent plants.

The F2 generation that is present when the plants self-pollinate is the second generation, and all the plants have variations of seed shape and seed color. In this example, about 9/16 of the plants have yellow seeds with a wrinkled shape. About 3/16 obtain green as the seed color and round as the shape. Approximately 3/16 get yellow seeds in color and the wrinkled shape and the remaining 1/16 get a green color seed with a wrinkled shape. The F2 generation exhibits four phenotypes and nine genotypes as a result.

What Is a Monohybrid Cross in Genetics?

A monohybrid genetic cross centers around only one trait that is different in the two parent plants. Both parent plants are homozygous for the studied trait, although they have different alleles for those traits. One parent is homozygous recessive and the other is homozygous dominant in the same trait. Just as in the dihybrid cross of plants, the F1 generation will be all heterozygous in a monohybrid cross. Only the dominant phenotype is observed in the F1 generation. But the F2 generation would be 3/4 of the dominant phenotype and 1/4 of the recessive phenotype that is observed.

What Are Heterozygous Mutations?

Genetic mutations can occur on chromosomes that permanently change the DNA sequence, so it is different than the sequence in most other people. Mutations can be as large as a segment of chromosomes with multiple genes or as small as a single pair of alleles. In a heredity mutation, the mutation is inherited and stays with the person in every cell of their body throughout their life.

The mutations occur when an egg and sperm cell unite and the fertilized egg gets DNA from both parents in which the resulting DNA has a genetic mutation. In diploid organisms, a mutation occurring on only one allele for a gene is a heterozygous mutation.

Gene Mutations and Affects on Health and Development

Each cell in the human body depends on thousands of proteins that must appear in the correct areas to do their jobs and promote healthy development. A gene mutation can prevent one or more proteins from proper functioning, and it can cause the malfunctioning of a protein, or it can be missing from the cell. These things that coincide with genetic mutations can disrupt normal development or cause a medical condition in the body. This is often referred to as a genetic disorder.

In the case of severe genetic mutations, an embryo may not even survive long enough to reach birth. This happens with genes that are essential for development are affected. Very serious gene mutations will be incompatible with life in any manner, so the embryo will not live until birth.

Genes do not cause disease, but a genetic disorder can make a gene fail to function properly. If someone says that a person has bad genes, it is actually a case of a defective or mutated gene.

What Are the Different Types of Gene Mutations?

Your DNA sequence can be altered in seven different manners to result in a gene mutation.

Missense mutation is the change in one base pair of DNA. It results in the substitution of one amino acid for a different one in the gene's protein.

Nonsense mutation is a change in the base pair of DNA. It does not substitute one amino acid for another, but instead the DNA sequence will prematurely signal a cell to stop making a protein resulting in a shortened protein that can function improperly or not at all.

Insertion mutations change the amount of DNA bases because they add an additional piece of DNA that does not belong. This can cause the gene's protein to malfunction.

Deletion mutations are the opposite of insertion mutation, as there is a piece of DNA that is removed. Deletions may be small with only a few base pairs affected, or they can be large when an entire gene or neighboring genes are deleted.

A duplication mutation is when a piece of DNA copies itself one or more times to result in improper functioning of the protein made by the mutation.

Frameshift mutations occur when a gene's reading frame is changed due to the loss or addition of DNA base changes. Reading frames contain groups of three bases with each code for one amino acid. A frameshift mutation shifts the groups of three and changes the amino acids codes. The protein as a result of this action is usually nonfunctional.

Repeat expansion mutations are when nucleotides repeat a number of times in a row. It basically increases the number of times that the short DNA gets repeated.

What Is a Compound Heterozygote?

A compound heterozygote occurs when there are two mutant alleles, one from each parent, in the pairs of genes in the same location. Both alleles have genetic mutations, but each allele in the pair has a different mutation. This is called compound heterozygote or the genetic compound, which involves both pairs of alleles in one area of the chromosome.

What Are Some Examples of Color Genetics in Dogs?

As a heterozygous example, every dog carries a set of two alleles at one location on a chromosome for their traits. Most often, one is recessive, and one is dominant, and the dominant color will appear for the coat color of puppies, as the phenotype. Look at Labrador retrievers, and their dominant colors, where the dominant color is black, and the recessive color is chocolate.

The dominant traits are expressed in capital letters and the recessive traits are expressed in lower case letter for the genotype. For example, a dog with the genotype of BB has two dominant alleles, and it will only express B, as both are dominant. A dog with Bb as a genotype will express B, as B is dominant, and b is recessive. The genotype of bb, with both being recessive will be the only genotype that expresses the b color.

References

About the Author

Mary Lougee has been writing about chemistry, biology, algebra, geometry, trigonometry and calculus for more than 12 years. She gained the knowledge in these fields by taking accelerated classes throughout college while gaining her degree.

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