homozygous

Examples of homozygous in the following topics:

• Hardy-Weinberg Principle of Equilibrium

  • The frequency of homozygous pp individuals is p2; the frequency of hereozygous pq individuals is 2pq; and the frequency of homozygous qq individuals is q2.
  • 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 homozygous dominant plants (p2) is (0.6)2 = 0.36.
  • Out of 100 individuals, there are 36 homozygous dominant (YY) plants.

• 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.
  • When the genes A and B are both homozygous recessive (aabb), the seeds are ovoid.
  • The recessive c allele does not produce pigmentnand a mouse with the homozygous recessive cc genotype is albino regardless of the allele present at the A locus.

• Lethal Inheritance Patterns

  • In one quarter of their offspring, we would expect to observe individuals that are homozygous recessive for the nonfunctional allele.
  • An inheritance pattern in which an allele is only lethal in the homozygous form and in which the heterozygote may be normal or have some altered non-lethal phenotype is referred to as recessive lethal.
  • 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.

• 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.
  • Because heterozygotes could arise from two different pathways (receiving one dominant and one recessive allele from either parent), and because heterozygotes and homozygous dominant individuals are phenotypically identical, the law supports Mendel's observed 3:1 phenotypic ratio.

• 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 Punnett Square Approach for a Monohybrid Cross

  • The dominant seed color is yellow; therefore, the parental genotypes were YY (homozygous dominant) for the plants with yellow seeds and yy (homozygous recessive) for the plants with green seeds, respectively.
  • 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.
  • In a test cross, the dominant-expressing organism is crossed with an organism that is homozygous recessive for the same characteristic.

• Mendel's Law of Independent Assortment

  • Because each parent is homozygous, the law of segregation indicates that the gametes for the green/wrinkled plant all are yr, while the gametes for the yellow/round plant are all YR.
  • For instance, for a tetrahybrid cross between individuals that are heterozygotes for all four genes, and in which all four genes are sorting independently in a dominant and recessive pattern, what proportion of the offspring will be expected to be homozygous recessive for all four alleles?
  • We know that for each gene the fraction of homozygous recessive offspring will be 1/4.
  • Therefore, multiplying this fraction for each of the four genes, (1/4) × (1/4) × (1/4) × (1/4), we determine that 1/256 of the offspring will be quadruply homozygous recessive.

• Mendel’s Model System

  • Today, we know that these "true-breeding" plants are homozygous for most traits.
  • In this and all the other pea plant traits Mendel followed, one form of the trait was "dominant" over another so it masked the presence of the other "recessive" form in the first generation after cross-breeding two homozygous plants..

• Sex-Linked Traits

  • With regard to Drosophila eye color, when the P1 male expresses the white-eye phenotype and the female is homozygous red-eyed, all members of the F1 generation exhibit red eyes.
  • 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.
  • Now, consider a cross between a homozygous white-eyed female and a male with red eyes.

• Alternatives to Dominance and Recessiveness

  • For example, in the snapdragon , Antirrhinum majus, a cross between a homozygous parent with white flowers (CWCW) and a homozygous parent with red flowers (CRCR) will produce offspring with pink flowers (CRCW).