Biology topic 10

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What does Mendel’s law of independent assortment state (discovered by doing dihybrid crosses)?
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. –just because 1 traits inherited doesn’t mean any other specific trait must be passed on as well.
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What is the law of segregation?
The two alleles of a gene separate into different haploid gametes during meiosis shown by Mendel’s monohybrid crosses with pea plants.
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What is the usual ratio in dihybrid crosses?
9:3:3:1 –the product of two 3:1 rations (two different genes).
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When can dihybrid crosses give other ratios?
If either of the genes has co-dominant alleles, either of the parents is homozygous for one or both genes or if either of the genes is autosomal (sex-linked). Another cause = epistasis.
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How can independent assortment of unlinked genes be explained in terms of chromosome movements during meiosis?
When pairing of homologous chromosomes occurs curing prophase I of meiosis (first stage), the alleles of unlinked genes are on different pairs of homologous chromosomes (bivalents).
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What can happen in prophase I of meiosis?
Homologous chromosomes pair up. While the chromosomes are paired, sections of chromatid are exchanged in a process called crossing-over (required to produce new combinations of linked gene
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Chiasma
X shaped structure formed when each cross-over has occurred
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Describe crossing over.
Synapsis =4 chromatids in total long and thin at this stage 2.DNA cut at same points in 2 non-sister chromatids 3. DNA of each chromatid joined up to DNA of non-sister chromatid 4.Synapsis ends but sister chromatids remain connected, chiasma formed.
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Describe discrete variation.
Every individual fits into 1 of a no. of non-overlapping classes i.e. blood group. It is usually due to one gene.
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Describe continuous variation.
When there’s an array of phenotypes between two extremes. If it’s genetically determined it’s due to the combined effects of 2 or more genes (polygenic inheritance).
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What’s the difference between gradualism and punctuated equilibrium?
Gradualism refers to speciation that occurs over thousands of year whereas punctuated equilibrium refers to long periods without appreciable change and short periods of rapid + abrupt evolution (sometimes due to dramatic change in environment).
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What are the three different types of reproductive isolation?
Temporal (when populations of a species breed at different times), Behavioural (when populations of a species have behaviours that prevent interbreeding), geographical (when populations of a species live in different areas + therefore don’t interbree
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Speciation can also occur by polyploidy. What is polyploidy?
When an error leads to an individual having more than 2 sets of chromosomes. Tertraploid = individual with 32 chromosomes in a species with a diploid no. of 16 –if this crosses with a diploid the offspring are infertile triploids.
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Unlinked genes
Genes that assort independently (when they’re located on different chromosomes)
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Gene linkage
The tendency of alleles that are located close together on a chromosome to be inherited together. Linked genes have loci on the same chromosomes.
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Bivalent
A pair of homologous chromosomes
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Locus
Location of a gene on a chromosome
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Recombination
When new combinations of the alleles of linked genes can only be produced if DNA is swapped between chromatids (crossing over)
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Recombinants
Individuals that have a different combination of characters from parents due to crossing over.
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Non-sister chromatids
Chromatids of two different chromosomes in a pair
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Synapsis
Stage in prophase I where all of the chromatids of 2 homologous chromosomes become tightly paired up together.
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Crossing over
The exchange of DNA material between non-sister homologous chromatids.
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Synapsis
stage in prophase 1 where 2 **** chromosomes become tightly paired up together
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Gene pool
A gene pool consists of all the genes + their different alleles in an interbreeding population.
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Reproductive isolation
When one population doesn’t interbreed with any other populations of its species eventually leading to speciation (if natural selection acts differently on this population it will gradually diverge from the other populations)
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Dihybrid crosses
Crosses in which the parents differ in 2 characteristics controlled by 2 different genes
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Phenotype ratio + Genotype ratio
Proportion of various phenotypes + proportion of various genotypes
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Epistasis
Interaction between genes
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Other cards in this set

Card 2

Front

What is the law of segregation?

Back

The two alleles of a gene separate into different haploid gametes during meiosis shown by Mendel’s monohybrid crosses with pea plants.

Card 3

Front

What is the usual ratio in dihybrid crosses?

Back

Preview of the front of card 3

Card 4

Front

When can dihybrid crosses give other ratios?

Back

Preview of the front of card 4

Card 5

Front

How can independent assortment of unlinked genes be explained in terms of chromosome movements during meiosis?

Back

Preview of the front of card 5
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