Transcript GENETEICS
I don’t want a white rabbitgive me a red rabbit for the NEW YEAR By : Tan Kok Hwee, Ong Lay Hua Lesson Objectives At the end of the lesson, you should be able to: 1. explain the inheritance concept of monohybrid 2. predict the characteristics of offspring that result from simple monohybrid crosses of various dominant and recessive traits 3. compare the expression of dominance and co-dominance incomplete Scenario Ah Beng’s father has just passed away, leaving Ah Beng to take charge of the family business - a rabbit breeding farm. Ah Beng has no idea how to feed the rabbits, let alone breed them. Over the last 40 years, Ah Beng’s father had developed a special breed of rabbit - a RED coloured one! The Year of the Rabbit is round the corner. Ah Beng hopes to make megabucks by selling RED coloured ‘Ang Pow’ rabbits. However, he has one problem: his father only left him one male RED rabbit! He needs to breed more RED rabbits. Oh no! How am I supposed to get more Red rabbits when I only have one left? Suddenly, Ah Beng remembered his dad saying that the RED colour of the rabbit was due to a dominant allele. Ah Beng : Aha! My biology teacher said that if you cross an animal displaying a dominant trait with an animal displaying a recessive trait, the babies will show the dominant trait. Ah Beng went through his ‘O’ Level textbook and came up with a chart. Ah Beng’s chart Parental phenotypes RED White Parental genotypes RR rr Gametes R R r r Random Fertilization F1 genotypes F1 phenotype Rr Rr Rr All Red Rr With the help of his chart, Ah Beng successfully bred 8 Ang Pow Rabbits. He sold 4 of them for $4000!!! Feeling like an entrepreneur, Ah Beng thought ‘Using my remaining Ang Pow Rabbits, if I mate the only male with the 3 females and if each female produces 8 rabbits, ... I will have 24 Ang Pow Bunnies and they will earn me $24000. I’m going to be RICH !!!’ Will Ah Beng be able to make $24000? YES NO Can you cross Rr with Rr and still get offspring which are all RED?? Think about it ... Click to retry question Yes, you are correct. Ah Beng is not going to make $24000 even if he gets 24 bunnies. Parental Phenotypes RED Red Parental genotypes Rr Rr Gametes R r RR Rr R r Random Fertilization F1 genotypes F1 phenotype Rr rr 3 Red : 1 White Only 18 out of every 24 rabbits will be Ang Pows. The rest will be albinos. So he will earn at most $18 000. Ah Beng soon become very famous in Singapore. One day, he received a phone call from a Miss Lao, offering to pay $3000 for each PINK rabbit he could produce. Ah Beng is keen to try. His former Biology teacher told him to check out incomplete dominance and codominance. Which one of these will generate the pink rabbits? Incomplete dominance Co-dominance Very Good! In incomplete dominance, neither allele is completely dominant over the other. Both Red and White alleles will exert their effects giving rise to an intermediate phenotype...... PINK! In co-dominance, both alleles express themselves equally. Offspring will show both traits. If that happens, your rabbit will look like this. Click here to see for yourself Can Ah Beng create such rabbits from his current stock? YES NO Incorrect. His current stock of rabbits are red even when the animals are heterozygous! Correct! The alleles controlling colour in his present stock of rabbits are either dominant (red) or recessive (albino). Created by: Tan Kok Hwee and Ong Lay Hwa Click to end Glossary: Gene : A hereditary factor carried on a particular locus in chromosomes which controls a particular trait. E.g. Gene for fur colour. Alleles : Different forms of the same gene. E.g. Allele for black fur colour, allele for white fur colour, etc. Dominant allele : The allele whose effect is always shown when it is present. E.g. If allele for black fur (B) is dominant over the allele for white fur (b), then the animal has black fur. Recessive allele : The allele whose effect is always masked when it is present with a dominant allele. Eg. Allele for white fur (b) in the above case. Genotype : The combination of genes in an organism. E.g. BB, Bb or bb. Phenotype: : The observable traits of an organism. E.g. Black fur colour, white fur colour. Homozygous : An organism with identical alleles for the same trait. E.g. BB or bb Heterozygous : An organism with different alleles for the same trait. E.g. Bb Co-dominance : Both alleles exert their effects equally such that the organism show s patches of ‘pure colour’. E.g. If allele B and allele b are co-dominant, a heterozygous animal will have patches of black and white fur Incomplete dominance : Both alleles exert their effects equally such that the organism shows an intermediate trait. E.g. If allele B and allele b demonstrate incomplete dominance, a heterozygous animal will have gray fur.