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IB DP Biology: HL復(fù)習(xí)筆記10.2.1 Unlinked Genes

Category: IB課程, 教材筆記, 福利干貨 Date: 2022年7月27日上午10:11

Independent Assortment & Segregation

Unlinked genes segregate independently as a result of meiosis

Unlinked genes?are genes that an organism carries on?separate chromosomes
- Not on homologous copies of the same chromosome
An example of a pair of unlinked genes in fruit flies (Drosophila melanogaster)?is
- The gene for curly wings on?chromosome 2, and
- The gene for mahogany eyes on?chromosome 3
An example of a pair of unlinked genes in humans?is
- The gene for?trypsin (a stomach enzyme) on chromosome?7,?and
- The gene for human growth hormone on chromosome?17
Assortment?of chromosomes refers to their?alignment in metaphase I of meiosis
- Each bivalent assorts (aligns) itself independently of all the others
Segregation?of chromosomes (ie. how they get separated) is governed by their pattern of assortment
- Segregation just refers to?which pole of the cell?the whole chromosomes are pulled to in anaphase I
- Segregation determines?which combinations of alleles?end up in which gamete cells by the end of meiosis II
By contrast, linked genes (on the same chromosome) tend to be?inherited together

The loci of selected genes in the human genome

Trypsin and CFTR are linked genes (both on the same chromosome);

Human Growth Hormone and trypsin are unlinked genes (both on different chromosomes)

Punnett Squares for Dihybrid Traits

Monohybrid crosses look at how the alleles of?one?gene transfer across generations
Dihybrid crosses look at how the alleles of?two?genestransfer across generations
- ie. dihybrid crosses can be used to show the?inheritance?of?two completely different characteristics?in an individual
The genetic diagrams for both types of cross are very similar
For dihybrid crosses, there are several more genotypes and phenotypes involved
When writing out the different genotypes, write the?two alleles for one gene, followed immediately by the?two alleles for the other gene.
Do not mix up the alleles from the different genes
- For example, if there was a gene with alleles?Y?and?yand another gene with alleles?G?and?gan example genotype for an individual would be?YyGg
Alleles are usually shown side by side in dihybrid crosses e.g.?TtBb

Worked Example

Worked example 1: Dihybrid genetic diagram

Horses have a single gene for coat colour that has two alleles:
- B, a dominant allele produces a black coat
- b, a recessive allele produces a chestnut coat
Horses also have a single gene for eye colour
- E, a dominant allele produces brown eyes
- e, a recessive allele produces blue eyes
Each of these genes (consisting of a pair of alleles) are?inherited independently?of one another because the two genes are located ondifferent non-homologous chromosomes
- Such characteristics are said to be unlinked
In this example, a horse that is heterozygous for both genes has been crossed with a horse that is homozygous for one gene and heterozygous for the other

Parental phenotypes:	black coat, brown eyes	x	chestnut coat, brown eyes
Parental genotypes:	BbEe	x	?bbEe
Parental gametes:	BE?or?Be?or?bE?or?be	x	bEor?be

determining-gamete-alleles-from-parental-genotypesdetermining-gamete-alleles-from-parental-genotypes

Determining the Alleles Carried by Gametes Based on the Parental Genotypes Using the FOIL (First, Outside, Inside, Last) Method

Dihybrid Cross Punnett Square Table

Predicted ratio of phenotypes in offspring =
- 3?black coat, brown eyes :
- 3?chestnut coat, brown eyes :
- 1?black coat, blue eyes :
- 1?chestnut coat, blue eyes
Predicted ratio of genotypes in offspring =?3?BbEE?: 3?bbEE?: 1?Bbee?: 1?bbee

Worked Example

Worked example 2: Dihybrid genetic diagram

In a separate cross to that shown in Worked Example 1, a horse that is heterozygous for both genes has been crossed with another horse that is heterozygous for both genes.

Parental phenotypes:	black coat, brown eyes	x	black coat, brown eyes
Parental genotypes:	BbEe	x	?BbEe
Parental gametes:	BE?or?Be?or?bE?or?be	x	BE?or?Be?or?bEor?be

Dihybrid Cross Punnett Square Table 2

dihybrid-cross-punnett-square-table-unlinked-genes-ib-

Predicted ratio of phenotypes in offspring =
- 9?black coat, brown eyes :
- 3?chestnut coat, brown eyes :
- 3?black coat, blue eyes :
- 1?chestnut coat, blue eyes

Exam Tip

For the double-heterozygous cross for unlinked genes above, you're expected to remember the phenotypic ratio 9:3:3:1. You won't need to remember the ratio of the genotypes but this can be worked out from a Punnett square like the one above.

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