If the genotype is "Aa, Bb", then 4 different kinds of gametes can be made: aB, AB, ab, and Ab A creature with 2 heterozygous genes is called a "dihybrid.
Genetic variation is increased by meiosis During fertilisation, 1 gamete from each parent combines to form a zygote. Because of recombination and independent assortment in meiosis, each gamete contains a different set of DNA. This produces a unique combination of genes in the resulting zygote.
That means that one person could produce 2 23 different gametes. What gametes can be produced by AaBb? There are four possible combinations of gametes for the AaBb parent. Half of the gametes get a dominant A and a dominant B allele; the other half of the gametes get a recessive a and a recessive b allele.
How do you calculate gametes? Simply place a 2 above each heterozygous gene pair and a one above each homozygous gene pair. Then multiply the numbers together to obtain the total number of different possible gametes.
How do you determine gametes? When gametes are produced, members of a gene pair separate into different gametes. Therefore, you must separate the genes Rr and the genes Tt when producing the gametes. The idea that genes were carried on cytological structures is now known as the chromosome theory.
Using such methods, researchers determined that although mitosis and meiosis are both forms of cell division , the results of these processes are actually quite different. Mitosis occurs in somatic cells; this means that it takes place in all types of cells that are not involved in the production of gametes. Prior to each mitotic division, a copy of every chromosome is created; thus, following division, a complete set of chromosomes is found in the nucleus of each new cell.
Indeed, apart from random mutations, each successive duplicate cell will have the same genetic composition as its parent, due to the inheritance of the same chromosome set and similar biological environment. This works well for replacing damaged tissue or for growth and expansion from an embryonic state. Because the genes contained in the duplicate chromosomes are transferred to each successive cellular generation, all mitotic progeny are genetically similar.
However, there are exceptions. For example, there are genetic variations that arise in clonal species , such as bacteria , due to spontaneous mutations during mitotic division. Furthermore, chromosomes are sometimes replicated multiple times without any accompanying cell division.
This occurs in the cells of Drosophila larvae salivary glands, for example, where there is a high metabolic demand. The chromosomes there are called polytene chromosomes, and they are extremely large compared to chromosomes in other Drosophila cells. These chromosomes replicate by undergoing the initial phases of mitosis without any cytokinesis Figure 2. Therefore, the same cell contains thick arrangements of duplicate chromosomes side by side, which look like strands of very thick rope.
Scientists believe that these chromosomes are hyper-replicated to allow for the rapid and copious production of certain proteins that help larval growth and metamorphosis Gilbert, The greatest impact of Sutton's work has far more to do with providing evidence for Mendel's principle of independent assortment than anything else.
Specifically, Sutton saw that the position of each chromosome at the midline during metaphase was random, and that there was never a consistent maternal or paternal side of the cell division. Therefore, each chromosome was independent of the other. Thus, when the parent cell separated into gametes, the set of chromosomes in each daughter cell could contain a mixture of the parental traits, but not necessarily the same mixture as in other daughter cells.
To illustrate this concept, consider the variety derived from just three hypothetical chromosome pairs, as shown in the following example Hirsch, Each pair consists of two homologues: one maternal and one paternal. Here, capital letters represent the maternal chromosome, and lowercase letters represent the paternal chromosome:. When these chromosome pairs are reshuffled through independent assortment , they can produce eight possible combinations in the resulting gametes:.
A mathematical calculation based on the number of chromosomes in an organism will also provide the number of possible combinations of chromosomes for each gamete. In particular, Sutton pointed out that the independence of each chromosome during meiosis means that there are 2 n possible combinations of chromosomes in gametes, with "n" being the number of chromosomes per gamete. Thus, in the previous example of three chromosome pairs, the calculation is 2 3 , which equals 8.
Furthermore, when you consider all the possible pairings of male and female gametes, the variation in zygotes is 2 n 2 , which results in some fairly large numbers. But what about chromosome reassortment in humans? Humans have 23 pairs of chromosomes. That means that one person could produce 2 23 different gametes. In addition, when you calculate the possible combinations that emerge from the pairing of an egg and a sperm, the result is 2 23 2 possible combinations.
However, some of these combinations produce the same genotype for example, several gametes can produce a heterozygous individual. Of course, there are more than 23 segregating units Hirsch, While calculations of the random assortment of chromosomes and the mixture of different gametes are impressive, random assortment is not the only source of variation that comes from meiosis.
In fact, these calculations are ideal numbers based on chromosomes that actually stay intact throughout the meiotic process. In reality, crossing-over between chromatids during prophase I of meiosis mixes up pieces of chromosomes between homologue pairs, a phenomenon called recombination.
Because recombination occurs every time gametes are formed, we can expect that it will always add to the possible genotypes predicted from the 2 n calculation. In addition, the variety of gametes becomes even more unpredictable and complex when we consider the contribution of gene linkage.
Some genes will always cosegregate into gametes if they are tightly linked, and they will therefore show a very low recombination rate. While linkage is a force that tends to reduce independent assortment of certain traits, recombination increases this assortment. In fact, recombination leads to an overall increase in the number of units that assort independently, and this increases variation. While in mitosis, genes are generally transferred faithfully from one cellular generation to the next; in meiosis and subsequent sexual reproduction , genes get mixed up.
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 How many of these 12 chromosomes will be contributed to the zygote at fertilization? How many of these 12 chromosomes will be contributed to the endosperm at fertilization? What is the diploid chromosome number of this hypothetical plant i.
What is the haploid chromosome number of this hypothetical plant i. What is the chromosome number of the endosperm tissue? What is the chromosome number of a polar nucleus inside the endosperm mother cell within the embryo sac?
A hypothetical angiosperm embryo sac megagametophyte contains a total of 32 chromosomes. Use the following answer choices for questions What is the chromosome number of an egg cell? What is the chromosome number of a synergid cell? How many total chromosomes are contained within the endosperm mother cell? How many nuclei are contained within this embryo sac? There are three main types of symbiosis, including commensalism, parasitism and mutualism. In commensalism , one organism in the relationship is benefited while the other is neither benefited nor harmed.
Some bird nests and trees form a commensal relationship. The birds obtain shelter and protection without harming the tree. Tutorial Predicting the genotype of offspring There are four possible combinations of gametes for the AaBb parent.
Half of the gametes get a dominant A and a dominant B allele; the other half of the gametes get a recessive a and a recessive b allele. Review problem 3's tutorial if necessary.
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