Punnett Square: Dominant and Recessive Traits | Science project | az-links.info
What's the difference between Dominant and Recessive? Genes determine traits, or characteristics, such as eye, skin, or hair color, of all Comparison chart. Dominance in genetics is a relationship between alleles of one gene, in which the effect on Thus, allele R is dominant to allele r, and allele r is recessive to allele R. This 6 Dominant and recessive genetic diseases in humans; 7 See also. The Punnett square is a square diagram that is used to predict the genotypes of a particular (It is conventional in genetics to use capital letters to indicate dominant alleles and lower-case letters to indicate recessive alleles.) The genotypic ratio was obtained in the diagram below, this diagram will have more branches.
An individual with one dominant and one recessive allele for a gene will have the dominant phenotype. 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. But the terms can be confusing when it comes to understanding how a gene specifies a trait.
This confusion comes about in part because people observed dominant and recessive inheritance patterns before anyone knew anything about DNA and genes, or how genes code for proteins that specify traits. The critical point to understand is that there is no universal mechanism by which dominant and recessive alleles act.
Whether an allele is dominant or recessive depends on the particulars of the proteins they code for. The terms can also be subjective, which adds to the confusion.
The same allele can be considered dominant or recessive, depending on how you look at it. The sickle-cell allele, described below, is a great example. However, these patterns apply to few traits. The sickle-cell allele Inheritance patterns Sickle-cell disease is an inherited condition that causes pain and damage to organs and muscles.
Dominant and Recessive Genes In Humans
Instead of having flattened, round red blood cells, people with the disease have stiff, sickle-shaped cells. The long, pointy blood cells get caught in capillaries, where they block blood flow. The disease has a recessive pattern of inheritance: People with just one copy are healthy. In addition to causing disease, the sickle-cell allele makes people who carry it resistant to malaria, a serious illness carried by mosquitos.
Malaria resistance has a dominant inheritance pattern: This is the very same allele that, in a recessive inheritance pattern, causes sickle-cell disease! People with two copies of the sickle-cell allele have many sickled red blood cells.
People with one sickle-cell allele and one normal allele have a small number of sickled cells, and their cells sickle more easily under certain conditions. So we could say that red blood cell shape has a co-dominant inheritance pattern. That is, individuals with one copy of each allele have an in-between phenotype. So is the sickle cell allele dominant, recessive, or co-dominant?
It depends on how you look at it. Protein function If we look at the proteins the two alleles code for, the picture becomes a little more clear. The affected protein is hemoglobin, the oxygen-carrying molecule that fills red blood cells. The sickle-cell allele codes for a slightly modified version of the hemoglobin protein.
The modified hemoglobin protein still carries oxygen, but under low-oxygen conditions the proteins stick together. When a person has two sickle cell alleles, all of their hemoglobin is the sticky form, and the proteins form very long, stiff fibers that distort red blood cells.
When someone has one sickle-cell allele and one normal allele, only some of the hemoglobin is sticky. When bred separately, the plants always produced the same phenotypes, generation after generation.
However, when lines with different phenotypes were crossed interbredone and only one of the parental phenotypes showed up in the offspring green, or round, or red, or tall. However, when these hybrid plants were crossed, the offspring plants showed the two original phenotypes, in a characteristic 3: Mendel reasoned that each parent in the first cross was a homozygote for different alleles one parent AA and the other parent aathat each contributed one allele to the offspring, with the result that all of these hybrids were heterozygotes Aaand that one of the two alleles in the hybrid cross dominated expression of the other: Mendel did not use the terms gene, allele, phenotype, genotype, homozygote, and heterozygote, all of which were introduced later.
He did introduce the notation of capital and lowercase letters for dominant and recessive alleles, respectively, still in use today. Chromosomes, genes, and alleles[ edit ] See also: Ploidy and Zygosity an autosomal dominant pattern. Most animals and some plants have paired chromosomesand are described as diploid. They have two versions of each chromosome, one contributed by the mother's ovumand the other by the father's spermknown as gametesdescribed as haploid, and created through meiosis.
Dominant and Recessive Genes In Humans - Science Brainwaves
These gametes then fuse during fertilization during sexual reproductioninto a new single cell zygotewhich divides multiple times, resulting in a new organism with the same number of pairs of chromosomes in each non-gamete cell as its parents.
Each chromosome of a matching homologous pair is structurally similar to the other, and has a very similar DNA sequence locisingular locus. The DNA in each chromosome functions as a series of discrete genes that influence various traits.
Thus, each gene also has a corresponding homologue, which may exist in different versions called alleles. The alleles at the same locus on the two homologous chromosomes may be identical or different. The blood type of a human is determined by a gene that creates an A, B, AB or O blood type and is located in the long arm of chromosome nine.
There are three different alleles that could be present at this locus, but only two can be present in any individual, one inherited from their mother and one from their father. The genetic makeup of an organism, either at a single locus or over all its genes collectively, is called its genotype.
The genotype of an organism directly and indirectly affects its molecular, physical, and other traits, which individually or collectively are called its phenotype. At heterozygous gene loci, the two alleles interact to produce the phenotype. Complete dominance[ edit ] In complete dominance, the effect of one allele in a heterozygous genotype completely masks the effect of the other.
The allele that masks the other is said to be dominant to the latter, and the allele that is masked is said to be recessive to the former. A classic example of dominance is the inheritance of seed shape pea shape in peas.
Peas may be round associated with allele R or wrinkled associated with allele r. In this case, three combinations of alleles genotypes are possible: RR and rr are homozygous and Rr is heterozygous. The RR individuals have round peas and the rr individuals have wrinkled peas. In Rr individuals the R allele masks the presence of the r allele, so these individuals also have round peas. Thus, allele R is completely dominant to allele r, and allele r is recessive to allele R.
Incomplete dominance[ edit ] This Punnett square illustrates incomplete dominance. In this example, the red petal trait associated with the R allele recombines with the white petal trait of the r allele. The plant incompletely expresses the dominant trait R causing plants with the Rr genotype to express flowers with less red pigment resulting in pink flowers. The colors are not blended together, the dominant trait is just expressed less strongly.
Incomplete dominance also called partial dominance, semi-dominance or intermediate inheritance occurs when the phenotype of the heterozygous genotype is distinct from and often intermediate to the phenotypes of the homozygous genotypes. For example, the snapdragon flower color is homozygous for either red or white. When the red homozygous flower is paired with the white homozygous flower, the result yields a pink snapdragon flower.
The pink snapdragon is the result of incomplete dominance. A similar type of incomplete dominance is found in the four o'clock plant wherein pink color is produced when true-bred parents of white and red flowers are crossed.
In quantitative geneticswhere phenotypes are measured and treated numerically, if a heterozygote's phenotype is exactly between numerically that of the two homozygotes, the phenotype is said to exhibit no dominance at all, i. When plants of the F1 generation are self-pollinated, the phenotypic and genotypic ratio of the F2 generation will be 1: This diagram shows co-dominance. In this example a white bull WW mates with a red cow RRand their offspring exhibit co-dominance expressing both white and red hairs.
Co-dominance occurs when the contributions of both alleles are visible in the phenotype. For example, in the ABO blood group systemchemical modifications to a glycoprotein the H antigen on the surfaces of blood cells are controlled by three alleles, two of which are co-dominant to each other IA, IB and dominant over the recessive i at the ABO locus.
The IA and IB alleles produce different modifications. The enzyme coded for by IA adds an N-acetylgalactosamine to the membrane-bound H antigen. The IB enzyme adds a galactose. The i allele produces no modification.
The medical condition produced by the heterozygous genotype is called sickle-cell trait and is a milder condition distinguishable from sickle-cell anemiathus the alleles show incomplete dominance with respect to anemia, see above. For most gene loci at the molecular level, both alleles are expressed co-dominantly, because both are transcribed into RNA. Co-dominance, where allelic products co-exist in the phenotype, is different from incomplete dominance, where the quantitative interaction of allele products produces an intermediate phenotype.
For example, in co-dominance, a red homozygous flower and a white homozygous flower will produce offspring that have red and white spots. These ratios are the same as those for incomplete dominance. Again, note that this classical terminology is inappropriate — in reality such cases should not be said to exhibit dominance at all.
Punnett square - Wikipedia
Addressing common misconceptions[ edit ] While it is often convenient to talk about a recessive allele or a dominant trait, dominance is not inherent to either an allele or its phenotype. Dominance is a relationship between two alleles of a gene and their associated phenotypes. A "dominant" allele is dominant to a particular allele of the same gene that can be inferred from the context, but it may be recessive to a third allele, and codominant to a fourth.
Similarly, a "recessive" trait is a trait associated with a particular recessive allele implied by the context, but that same trait may occur in a different context where it is due to some other gene and a dominant allele. Dominance is unrelated to the nature of the phenotype itself, that is, whether it is regarded as "normal" or "abnormal," "standard" or "nonstandard," "healthy" or "diseased," "stronger" or "weaker," or more or less extreme.
A dominant or recessive allele may account for any of these trait types. Dominance does not determine whether an allele is deleterious, neutral or advantageous.