If a genotype has two different alleles (letters), tt, it is said to be what?

Heterozygous, as related to genetics, refers to having inherited different versions (alleles) of a genomic marker from each biological parent. Thus, an individual who is heterozygous for a genomic marker has two different versions of that marker. By contrast, an individual who is homozygous for a marker has identical versions of that marker.

Narration

Heterozygous. In diploid species, there are two alleles for each trait of genes in each pair of chromosomes, one coming from the father and one from the mother. An allele is one of two or more alternative forms of a gene, and they are found at the same place, or locus, on the chromosome. Heterozygous refers to having different alleles for a particular trait. If the two versions are different, you have a heterozygous genotype for that gene. The relationship between the two alleles affects which traits are expressed.

• Since human cells carry two copies of each chromosome they have two versions of each gene. These different versions of a gene are called alleles.
• Alleles can be either dominant or recessive.
• Dominant alleles show their effect even if the individual only has one copy of the allele (also known as being heterozygous). For example, the allele for brown eyes is dominant, therefore you only need one copy of the ‘brown eye’ allele to have brown eyes (although, with two copies you will still have brown eyes).
• If both alleles are dominant, it is called codominance. The resulting characteristic is due to both alleles being expressed equally. An example of this is the blood group AB which is the result of codominance of the A and B dominant alleles.
• Recessive alleles only show their effect if the individual has two copies of the allele (also known as being homozygous). For example, the allele for blue eyes is recessive, therefore to have blue eyes you need to have two copies of the ‘blue eye’ allele.

Illustration to show the inheritance of dominant and recessive alleles for eye colour.
Image credit: Genome Research Limited

What are sex-linked genes?

• Some genes are found on the sex chromosome, X.
• These genes are inherited with the X chromosome (from the mother if it is a boy or from either mother or father if it is a girl).
• Females have two X chromosomes (XX), while males have one X chromosome and one Y chromosome (XY).
• This means females have two alleles for X-linked genes while males only have one.
• Some genetic diseases, are caused by sex linked genes, for example haemophilia.
• The allele for haemophilia is recessive so two copies are needed for a female to have the disease
• However, because males only have one X chromosome, they only need one copy of the haemophilia allele to have the disease.
• This means haemophilia is much more common in males than in females.

For example:

Functioning allele = H

Haemophilia allele = h

XH XH = healthy female

XH Xh = carrier female

Xh Xh = haemophilia female

XH Y = healthy male

Xh Y = haemophilia male

How helpful was this page?

👎 👍 Send

What's the main reason for your rating?

Send

Which of these best describes your occupation?

Send

how old are students / how old are you?

Send

What is the first part of your school's postcode?

Send

How has the site influenced you (or others)?

Send

Thankyou, we value your feedback!

If you have any other comments or suggestions, please let us know at [email protected]

Feedback

Can you spare 5-8 minutes to tell us what you think of this website? Open survey

Primer of Mendelian Genetics

    The appearance of an organism (phenotype) is influenced by its hereditary makeup (genotype).  Many individual characters or traits (morphological, behavioral, biochemical, molecular, etc.) of organisms are influenced more or less directly by individual hereditary elements called genes. Genes are located on chromosomes, each at a particular physical location called a locus (plural, loci).

     Genetics is the science of analyzing phenotypes to infer the nature of their underlying genotypes. The basic principles were first described by Gregor Mendel in 1867. Genetics operated as a distinct science after the rediscovery of Mendel's work in 1900, without knowledge of the physical hereditary material, until the discovery of DNA structure in 1953. Genetics is distinct from molecular biology, which analyzes genotypes (in a DNA molecule) to predict phenotypes (which are often direct or indirect products of proteins). For this reason, the so-called Central Dogma of molecular biology (DNARNA Protein) is sometimes called "reverse genetics."

1. Alternative forms of genes are called alleles; every individual possesses two alleles for each gene*.
    An individual with two identical alleles is called a homozygote and is described as homozygous;
        an individual with two dissimilar alleles is called a heterozygote and is described as heterozygous.

2. Some alleles (called dominant) 'mask' the phenotypic expression of other alleles (called recessive).
    Dominance is determined by comparison of the heterozygous phenotype with that of the two homozygotes
         Whichever of the two homozygotes the heterozygote resembles most closely determines the dominant allele
            'Dominance' is a genetic relationship, unrelated to frequency (common or rare) or phenotypic effect of the allele
    Dominant alleles are symbolized with a capital letter (A);
        recessive alleles are symbolized with a lower-case letter (a).

    For example: some people can taste the chemical phenylthiocarbimide (PTC) ("tasters"),
            and some cannot ("non-tasters").
        The character "PTC sensitivity" is influenced by a gene with two alleles,
             one associated with "taster" and one with "non-taster".
        The "taster" allele masks the expression of the  "non-taster" allele in heterozygotes:
             Homozygous TT or heterozygous Tt individuals both show the "T" phenotype ("taster"):
             only a homozygous tt individual show the "t" phenotype ("non-taster").
         Because the phenotype of the Tt individuals resembles that of the TT individuals,
              the T allele is described as dominant to the t allele.

3. The two alleles separate (segregate) during the formation of gametes (eggs & sperm);
     half of the germs cells carry one allele & half carry the other [Mendel's Law of Segregation].

4. Random union of gametes produces zygotes that develop into new individuals.
    Zygotic genotypes occur in characteristic ratios, according to the genotypes of the parents.
    For example, a cross between two heterozygotes (Aa x Aa)
        produces an expected genotypic ratio of 1:2:1 among AA, Aa, & aa genotypes.

5. The genotypic ratios produce characteristic phenotypic ratios,
        according to the dominance relationships of the alleles involved.
        For example, if A is dominant to a, the cross between heterozygotes produces
            an expected phenotypic ratio of 3:1 among "A" and "a" phenotypes.

6.  Alleles at separate loci are inherited independently [Mendel's Law of Independent Assortment]
       This produces characteristic genotypic and phenotypic ratios among multiple loci.
           For example, in a dihybrid cross between two "double heterozygotes" ( AaBb x AaBb )
                The genotypic ratios are 1 : 2 : 1 : 2 : 4 : 2 : 1 : 2 : 1
                    for the genotypes AABB  AABb  AAbb  AaBB  AaBb  Aabb aaBB  aaBb  aabb
                and the phenotypic ratios are 9 "AB" : 3 "Ab" : 3 "aB" : 1 "ab"

* Mendel was unaware that genes reside on chromosomes
      Genes that occur on the same chromosome are said to be linked
      Gene loci located near each other on a single chromosome will not assort independently.
             The characteristic ratios will be modified, according to how close they are.
             The modified ratios can be used to create a genetic map of the chromosome

    For example, sex in humans is determined by genes on sex chromosomes (X and Y)
           females are XX have two alleles (one on each X)
           males are XY and have only one allele on the single X (hemizygous)
    Characters on the X (or Y) chromosomes are sex-linked

---

---

What is the genotype of TT?

Is TT a genotype or gamete?

Is TT a hybrid genotype?

When the two alleles in a pair are identical example TT or Tt the genotype is said to be?