IB DP Biology: HL復習筆記10.2.1 Unlinked Genes
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 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
- 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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