Mary and John are expecting a baby. Mary is heterozygous (Cc) for cystic fibrosi
ID: 61152 • Letter: M
Question
Mary and John are expecting a baby. Mary is heterozygous (Cc) for cystic fibrosis, and has two wild type alleles for sickle cell disease. John is heterozygous (Ss) for sickle cell disease, and has two wild type alleles for cystic fibrosis. Both conditions have an autosomal recessive mode of inheritance, and the genes are located on two different chromosomes. Using one or more Punnett squares, calculate the probability that the baby will carry one allele for cystic fibrosis AND one allele for sickle cell disease.
Referring to the previous question, imagine that both Mary and John are heterozygous for cystic fibrosis AND sickle cell disease. Using one or more Punnett squares, calculate the probability that the baby will express both conditions.
Explanation / Answer
The wild type or dominant allele for cystic fibrosis (CF) is designated as 'C'; and the recessive allele which encodes for cystic fibrosis is designated as 'c'. When both alleles are recessive (cc), the individual expresses cystic fibrosis.
The wild type or dominant allele for sickle cell disease (SCD) is designated as 'S' and the recessive allele for sickle cell disease as 's'. When both alleles are recessive (ss), the individual expresses sickle cell disease.
The genes for cystic fibrosis and sickle cell disease are located on different chromosomes, the alleles for each gene are independently assorted in the gametes, and will be inherited independently of each other.
1. Mary is heterozygous for cystic fibrosis and homozygous dominant for sickle cell disease. Thus, the genotype for Mary is CcSS
John homozygous dominant for cystic fibrosis and heterozygous for sickle cell disease. So, the genotype for John is CCSs
A cross between Mary and John can be shown as below.
F1 progeny from the above gametes can be depicted with the help of Punette's square as below.
CCSS Homozygous dominant for both CF and SCD
Thus, the probability that the baby has one recessive allele for cystic fibrosis and one recessive allele for sickle cell disease (i.e. heterozygous for both CF and SCD) is 25 %.
2. A cross between Mary and John, when they both are heterozygous for CF and SCD, the following offpsrings will be produced in F1 progeny.
Phenotypic ratio: 9 normal babies (homozygous or heterozygous dominant for CF & SCD): 3 babies expressing CF but normal for SCD: 3 babies expressing SCD and normal for CF: 1 baby expressing CF and SCD
The above cross represents Mendelian dihybrid cross, which gives 9:3:3:1 phenotypic ratio when two heterozygous individuals are crossed.
For CF and SCD phenotype to be expressed, both alleles for each gene should be recessive, i.e. the genotype of an offspring to express both CF and SCD would be 'ccss'.
The F1 progeny for cross between heterozygous Mary and heterozygous John has one offspring out of 16, with the genotype ccss. That is the offspring is autosomal recessive for cystic fibrosis as well as sickle cell disease.
Thus, the probability that the baby will express both conditions is 1/16 or 6.25 %.
Parent Genotype CcSS CCSs Gametes CS cS CS Cs