10.1 and 10.2 Flashcards
Characteristics of homologous chromosomes
Homologous Chromosomes means that the chromosomes are the same length and they have their centromeres in the same position. They also generally contain the same genes stored at the same loci.
During prophase I of meiosis, the process of synapsis brings together two homologous chromosomes in a pair called a bivalent.
The homologous chromosomes split in Meiosis I.
The major difference between them is that one chromosome in the bivalent came from the person’s mother and the other chromosome in the bivalent came from the person’s father.
Multiple genes
The idea of multiple genes is derived from Mendel’s law of independent assortment. The law of independent assortment ultimately claims that one allele a gamete receives does not impact the other. In relation to the pea plants, the ratio was 9:3:3:1, which Mendel found to be the expected result if each gamete was randomly selected.
An exception to this theory is linked genes: if genes are linked and only inherited together, then they are not independent.
Multiple genes working together- linked genes and interactions between genes in the genome
DNA Replication in S phase (interphase)
There is no interphase and S phase before meiosis II
Before meiosis can occur, the cell must prepare for cell division during interphase. An important step in this preparation is DNA replication, enabling the cell to have a copy of each chromosome.
Two pairs of single chromosomes are being replicated. After replication during interphase, there are still two pairs of chromosomes, the difference is that now they have two full copies of each chromosome connected at the centromere.
Homologous chromosomes in DNA replication: As each parent can have different alleles for each of the genes, along the chromatids the two homologous chromosomes are by no means identical. On the other hand, recall that the two sister chromatids for the same chromosome should be identical because they are the result of DNA replication during interphase.
Phases of meiosis
Interphase Prophase Metaphase Anaphase Telophase and cytokinesis
Meiosis II
Prophase II
Metaphase II
Anaphase II
Telophase II and cytokinesis
Mitosis and Meiosis
During anaphase I in meiosis, the sister chromatids stay together while they do not in Mitosis or meiosis II. In Anaphase I of Meiosis, the bivalents in meiosis are separated so that each homologous chromosome is pulled to opposite ends of the cell. Also Mitosis creates identical daughter cells while meiosis creates four varied haploid cells.
When chromosomes fail to separate it is called non-distinction
Variation in meiosis
Mixing genetic material between non-sister chromatids, in other worlds between paternal and maternal chromosomes, can occur when the chromatids intertwine and break. In order for crossing over to function correctly, identical breaks must occur at exactly the same position in adjacent non-sister chromatids.
The main ways in which gamete production is able to generate genetic variety in offspring are : Crossing over during prophase I and Random Orientation during metaphase I. Random orientation refers to the random lineup of homologous chromosomes.
Independent assortment:
Gregor Mendel’s law of independent assortment states that when gametes are formed, the separation of one pair of alleles between the daughter cells is independent of the separation of another pair of alleles. The law of independent assortment implies that alleles that determine different characteristics will be transmitted independently to the next generation.
As with many rules, there are exceptions. Some genes do, in fact, go hand-in-hand, so that one is placed in a gamete during meiosis with the other followers.
Description of chiasmata in crossing-over
Crossing over= Chiasmata
Crossing over occurs between homologous chromosomes
For crossing over to happen, each chromatid involved has to have a separated tip. Two segments each connect to the corresponding position on the other chromatid. The two tips are thus switched and each resulting chromatid has a segment of the other’s genetic material. The place where the two connect to each other is called a chiasma.
The process of crossing over happens when two non-sister chromatids swap segments of their DNA. This means that the maternal chromosome can end up with a segment of genetic material from a paternal chromosome, and vice versa. Thus, a chromosome originally carrying a recessive allele could end up with a dominant allele that was traded during crossing over.
Autosomes vs. sex chromosomes
Autosomes=22
Sex chromosomes: 2 sex chromosomes. (X or Y)
A trait or gene that is said to be sex-linked must have its locus on a sex chromosome.
Where a gene is located determines whether or not the trait it controls is more common in males or females.
When a trait is more common in one sex than the other, the chances are that the trat is sex linked, and that the locus of the gene is one either the X chromosme, the Y chromosme or both. If there is no pattern the frequency of a trait between females and males is probably an autosomal trait.
Linked Genes & Probability
As Morgan and his team demonstrated with their experiments on Drosophila, any two genes that are found on the same chromosome are said to be linked to each other. Linked genes are usually passed on to the next generation together.
A group of genes inherited together because they are found on the same chromosome are considered to be members of a linkage group. This applies to genes on both autosomal and sex chromosomes.
For example: in fruit flies, GGLL= genotype of a gray-bodied, long-winged parent
ggll= genotype of a black bodied shot winged parent..
There is nothing in the genotype notation GGLL that shows that G must be inherited with L. In order to show linkage, the following notation is used.
G L
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G L
The two horizontal bars symbolize homologous chromosomes and show that the locus of G is on the same chromosome as L.
A cross between a homozygous dominant true-breeding fruit fly (GGLL) and a homozygous recessive true-breeding fruit fly (ggll) will result in flies that are all heterozygous for both of the traits (GgLl). The flies will all be gray with long wings, but they will all be carriers for the recessive alleles.
Researchers in the lab would not be able to determine the genotypes of any particular fly in the jar just by looking at it. A test cross with a known homozygous recessive would be necessary to determine whether a fly’s phenotype is the result of a homozygous or heterozygous genotype.
The Linked genes in the heterozygote is G L
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g l
The test cross is done by mating an unknown fly (here heterozygous for both traits) with another that is known to be homozygous recessive for both traits g l
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g l
The cross between these two can be done through a punnett square:
By examining the alleles closely, it is possible to see that these offspring are different from either parent. A new shuffling of the alleles has created a new combination that does not match either of the parent’s genotype. The term recombinant is used to describe both the new chromosomes and the resulting organism. The recombinants are a result of crossing over. Without crossing over the allele G would always be inherited with L.
▪When gametes are made from the resulting bivalent shown on the right in the example, two will contain combinations found in the parents (either GL or gl) whereas two will contain recombinants (Gl and gL). Thus, even in linked genes, nature has found a way to increase variety through crossing over. ▪Outcomes of genetic crosses should typically follow Mendelian ratios of 3:1 for an F2 monohybrid cross or 9:3:3:1 for an F2 dihybrid cross. There is nothing alarming about a slight variation from these expended values, but if there is a significant deviation it suggests that independent assortment is not happening and, instead, the genes for the traits being observed are linked.
for more check out the study guide
Polygenic Inheritance
Multiple genes contribute to the expression of genes
Polygenic inheritance involves two or more genes influencing the expression of one trait. With two or more allelic pairs found at different loci, the number of possible genotypes is greatly increased.
Polygenic is leading to continuous- a range of expression. When there are many intermediate possibilities, then the trait shows continuous variation.
Continuous vs. Discontinuous traits
When multiple alleles are introduced the number of possibilities for a single trait increases accordingly. When a second gene is introduced, the number of possible genotypes increases dramatically. When an array of possible phenotypes can be produced it is called continuous variation. In humans continuous variation can also be seen in the genetic components of traits such as height, body shape, and intellectual aptitude. Each of these is influenced by environmental components.
When variation is not continuous it is referred to as discontinuous variation. The data for discontinuous variation can be displayed as bar charts. Blood type is an example of discontinuous variation.
Chi-Square Analysis
The chi squared test helps us to see statistically whether or not there is a good fit between a theoretical model and what really happens in nature. The chi-squared test should only be used if the size of the sample observed is greater than 30. Ideally, 50 or more observed values would be better.
