heterozygous
(adjective)
of an organism which has two different alleles of a given gene
Examples of heterozygous in the following topics:
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Sex Determination
- Individuals homozygous for X (XX) are female, while heterozygous individuals (XY) are male.
- Homozygous for Z (ZZ) results in a male, while heterozygous (ZW) results in a female.
- The presence of X and Y chromosomes are one of the factors responsible for sex determination in mammals, with males being the heterozygous sex.
- In birds, Z and W chromosomes determine sex, with females being the heterozygous sex.
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Lethal Inheritance Patterns
- However, consider two heterozygous parents that have a genotype of wild-type/nonfunctional mutant for a hypothetical essential gene.
- For crosses between heterozygous individuals with a recessive lethal allele that causes death before birth when homozygous, only wild-type homozygotes and heterozygotes would be observed.
- People who are heterozygous for the dominant Huntington allele (Hh) will inevitably develop the fatal disease.
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Hardy-Weinberg Principle of Equilibrium
- In our example, the possible genotypes are homozygous dominant (YY), heterozygous (Yy), and homozygous recessive (yy).
- We do not know how many are homozygous dominant (Yy) or heterozygous (Yy), but we do know that 16 of them are homozygous recessive (yy).
- The frequency of heterozygous plants (2pq) is 2(0.6)(0.4) = 0.48.
- Therefore, 48 out of 100 plants are heterozygous yellow (Yy).
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The Punnett Square Approach for a Monohybrid Cross
- A self-cross of one of the Yy heterozygous offspring can be represented in a 2 × 2 Punnett square because each parent can donate one of two different alleles.
- Therefore, the two possible heterozygous combinations produce offspring that are genotypically and phenotypically identical despite their dominant and recessive alleles deriving from different parents.
- Beyond predicting the offspring of a cross between known homozygous or heterozygous parents, Mendel also developed a way to determine whether an organism that expressed a dominant trait was a heterozygote or a homozygote.
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Mendel's Laws of Heredity
- If the two alleles are different, the individual is called heterozygous.
- In heterozygous individuals, the only allele that is expressed is the dominant.
- He did this by cross-breeding dihybrids; that is, plants that were heterozygous for the alleles controlling two different traits.
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Epistasis
- Homozygous recessive expression of the W gene (ww) coupled with homozygous dominant or heterozygous expression of the Y gene (YY or Yy) generates yellow fruit, while the wwyy genotype produces green fruit.
- However, if a dominant copy of the W gene is present in the homozygous or heterozygous form, the summer squash will produce white fruit regardless of the Y alleles.
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Genes as the Unit of Heredity
- There can be many alleles of a gene within a population, but an individual within that population only has two copies and can be homozygous (both copies the same) or heterozygous (the two copies are different) for any given gene.
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Mendel's Law of Segregation
- For the F2 generation of a monohybrid cross, the following three possible combinations of genotypes could result: homozygous dominant, heterozygous, or homozygous recessive.
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Sex-Linked Traits
- The F1 females are heterozygous (XWXw), and the males are all XWY, having received their X chromosome from the homozygous dominant P1 female and their Y chromosome from the P1 male.
- The F1 generation would exhibit only heterozygous red-eyed females (XWXw) and only white-eyed males (XwY).
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Alternatives to Dominance and Recessiveness
- Such heterozygous individuals are sometimes referred to as "carriers."