With alleles 'A' and 'a' there are three possible genotypes AA, Aa and aa. With three alleles 1, 2, 3 there are six possible genotypes: 11, 12, 13, 22, 23, First we must appreciate that genes do not act in isolation. The genome in which a genotype is found can affect the expression of that genotype, and the environment can affect the phenotype.
An allele can be dominant over one allele but recessive to another allele. Model of dominance from enzyme activity: no copies produce no phenotype, one copy produces x amount of product and two copies produces 2x then the alleles are additive and there is no dominance intermediate inheritance.
If one copy of the allele produces as much product or has as high a rate of flux as a homozygote then there is dominance. There are cases where the heterozygote is greater in phenotypic value than either homozygote: called overdominance.
Single genes do not always work as simply as indicated by a dominance and recessive relationship. Other genes can affect the phenotypic expression of a given gene. One example is epistasis "standing on" where one locus can mask the expression of another. Classic example is a synthetic pathway of a pigment.
Mutations at loci controlling the early steps in the pathway gene 1 can be epistatic on the expression of genes later in the pathway gene 3 by failing to produce pigment precursors e. Genes can also be pleitropic when they affect more than one trait. Three haploid antipodal cells 1 occur at the upper end of the emryo sac. A large endosperm mother cell containing two haploid polar nuclei 2 occupies the central portion of the embryo sac.
At the lower end nearest the micropyle and funiculus are two haploid synergid cells 3 and one haploid egg. The embryo sac contains a total of seven cells and eight nuclei. The sporophyte generation includes the stems, leaves, roots, flowers, fruits and seeds.
After fertilization, the ovule enlarges and develops into a mature seed containing a diploid zygote and triploid endosperm. The seed coat is chromosomally identical to the female parent ovary tissue because it was derived from two outer layers of the ovule called the integument. On a mature seed the opening or pore in the seed coat is where the pollen tube once entered a gap in the integument layers called the micropyle.
As the ovules develop into seeds, the outer ovary encasing the ovules develops and ripens into a fruit. Fruits that develop without double fertilization and without seeds are termed parthenocarpic. Examples of parthenocarpic fruits are navel oranges, bananas, seedless watermelons, and certain varieties of figs. Not all seedless fruits are parthenocarpic. In Thompson seedless grapes, fertilization does occur, but the ovules fail to develop within the fruit. Parthenocarpy can be induced artificially by the application of dilute growth hormone sprays such as auxins to the flowers, as in seedless tomatoes.
Seedless watermelons come from triploid sterile plants; however, to set fruit they must be pollinated by a fertile diploid plant. Some embryos of seeds can develop apomictically without fertilization. A number of angiosperm families contain apomictic species, including figs, blackberries, hawthorns and dandelions.
The embryo may develop from a diploid nutritive cell nucellus tissue surrounding the embryo sac or from the fusion of hapolid cells within the embryo sac. In general there are two main types of apomixis: [1] Parthenogenesis agamogenesis : A haploid or diploid egg within the embryo sac or diploid cell from 2 fused haploid cells of embryo sac develops into an embryo.
If this involves cells of the nucellus or inner integument it is called a nucellar embryo. Nucellar embryos are chromosomally identical to the sporophyte parent. They are essentially clones of the female parent. In varieties of the edible fig Ficus carica , apomictic seeds allow propagation of choice edible fig cultivars female trees without the transmission of viruses through cuttings. Apomixis also enables a pioneer seedling to colonize and become naturalized in a new habitat by reseeding itself without cross pollination.
If the diploid pollen parent is aabbcc, then the haploid sperm would be [abc]. Since the seed coat originates from the outer wall of the ovule called the integument , which was part of the original maternal seed parent, it is chromosomally identical with the original diploid seed parent. The following diagram summarizes double fertilization in this question: Sperm 1 abc fuses with a haploid egg ABC resulting in a diploid zygote AaBbCc that grows into a diploid embryo AaBbCc within the seed.
The following remarkable Wayne's Word image shows a minute diploid coconut embryo embedded in the triploid, meaty endosperm within the seed of a coconut palm. Close-up view through the inside of a coconut seed showing a small, cylindrical embryo A embedded in the fleshy meat or endosperm B. The wall of the endocarp C is a hard, woody layer that makes up the inner part of the fruit wall. The thick, fibrous husk mesocarp that surrounds the endocarp has been removed.
A Note For Biology Students: In exalbuminous seeds, such as lima beans and walnuts, the endosperm has been completely absorbed by the embryo. The embryo of these seeds consists of two prominent halves called cotyledons. Attached between the cotyledons is a minute, primordial, leaf-bearing shoot called the plumule and an elongate primordial root called the radicle. See following photo: The embryo of a lima bean seed showing the embryonic shoot or plumule A , the embryonic root or radicle B and two cotyledons C.
The two fleshy halves called cotyledons are actually part of the embryo. The seed coat D has been partially removed from the cotyledons. Gametophyte refers to the chromosone number of gametes and sporophyte refers to the chromosome number of cells in adult plants. In humans, haploid n refers to the chromosome number of gametes, while diploid 2n refers to the chromosome number after fertilization; however, in polyploid plants the chromosome numbers are very different.
For example, I have studied a rare hybrid Brodiaea in San Marcos with a sporophyte chromosome number of The hybrid was derived from a cross between B. In this case, referring to the gametes as haploid n and the hybrid offspring as diploid 2n , as we do in humans, would be incorrect. In marriages between normal parents who produce a PKU child, the parents must be carriers heterozygous for the recessive gene causing this disease. Use the following Punnet Square genetic checkerboard to calculate the chance of a parent being heterozygous Aa and the fractional ratio of the deleterious recessive gene a for PKU: Don't forget to convert the fractional value for the frequency of the recessive gene, and the fractional probability of a couple having a PKU child, into percent values in order to find the correct answer on your ScanTron answer sheet.
Angiosperm Male Gametophyte Questions: A mature angiosperm pollen grain contains a tube nucleus and a generative nucleus, the latter of which divides into 2 sperm within the pollen tube. For this question, a hypothetical sperm-bearing angiosperm pollen tube contains a total of 12 chromosomes.
Use the following answer choices for questions a 0 b 4 c 6 d 8 e 12 In this allelic series, the alleles IA and IB each determine a unique antigen, which is deposited on the surface of the red blood cells. These antigens are two different forms of a single protein. What are allele combinations? Allele Combination. MGI Glossary. A designation of the specific alleles present on the two homologous chromosomes for all relevant loci of a mouse i.
An allele combination can be composed of one or more allele pairs. Where do alleles come from? How are new alleles created? Occasionally, DNA mutations occur in germ cells — cells destined to become eggs or sperm.
In this case, the DNA mutation is copied into every new cell of the growing embryo following fertilisation. Summarizing the Role of Dominance and Recessivity. References and Recommended Reading Keeton, W. Heredity 35 , 85—98 Parsons, P. Nature , 7—12 link to article Stratton, F. Article History Close. Share Cancel. Revoke Cancel. Keywords Keywords for this Article. Save Cancel. Flag Inappropriate The Content is: Objectionable. Flag Content Cancel. Email your Friend. Submit Cancel.
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