Why is the incidence of Duchenne’s Muscular Dystrophy highest in males?

A few human traits have easily identifiable patterns of inheritance. These are called “Mendelian traits” because they follow the rules of inheritance established by Johann Gregor Mendel in 1865. Mendel studied garden peas, a plant that usually self-fertilize because both the pollen containing the male gametes and the pistil containing the female gametes are enclosed by the petals of the pea blossom. Mendel cross-fertilized purple flowering pea plants with white flowering pea plants. He called the plants he started with the parental (P) generation. Those plants were true–breeding or homozygous for their color trait. The purple flowering pea plant contained two genes for the purple color (PP) and the white flowering pea plant contained two genes for the white color (pp). He found that all offspring of that cross-fertilization experiment were purple. When he left those purple pea plants (which he called the F1 generation) to self-fertilize, they produced purple and white flowers in a ratio of 3:1 in the next generation (the F2 generation). He reasoned that the white trait must have been present in the purple flowering plant of the F1 generation, although invisible. The plants of the F1 generation were heterozygous(Pp): they contained a gene for the purple trait (P) and a gene for the white trait (p). The color (what you see) is the phenotype. The genetic makeup (PP, Pp or pp) is the genotype. Mendel called the purple trait dominant and the white trait recessive. A white flower only occurs when two copies of the recessive white gene are present in the plant. Mendel’s results are represented in a Punnett square shown below:





Punnett square of a

monohybrid cross of

two heterozygous pea plants.







In the following exercise, you will learn to draw several pedigrees.



1. To understand the basics of Mendelian Genetics

2. To be able to generate various pedigrees



Punnett Square


Mr. and Mrs. Waverly both have curly hair. Five of their children have curly hair. One child has straight hair, just likely Grandma Waverly.

a. Which trait appears to be dominant? ____________________________________

b. Which trait seems to be recessive? ______________________________________

c. What are the probable genotypes of the entire Waverly family mentioned above? __________________________________________________________________

d. What is the probability of the Waverly’s NEXT child having straight hair? ___________________________________________________________________





Faced with the above situation, how can you go about answering the questions? Read the ENTIRE section BEFORE trying to tackle the problem.


Look At The Facts –

1. People are DIPLOID organisms.

We possess two sets of chromosomes.

These sets are homologous in nature, except for the X and Y chromosome.

22 of the chromosomes are considered AUTOSOMES.

They are numbered 1 – 22. You get a set of #1-22 from EACH of

your parents. 22 autosomes from Mom and 22 autosomes from Dad.

The other two chromatin strands are called SEX CHROMOSOMES.

Sex chromosomes are either an “X” or a “Y” chromososme.

44 autosomes plus XX = 46 chromosomes for a woman.

44 autosomes plus XY = 46 chromosomes for a man.


2. The homologous chromosomes have genes for the same trait located at the same position along their DNA molecule. Genes may have slight variations but are represented by the same letters – for example A, a, G, g, H, h.

Variations in alleles may be indicated by upper or lower case letters.




Homologous Chromosomes


3. Alleles are alternate forms a gene.

‘R’ and ‘r’ are alleles. Refer to the diagram of homologous chromosomes. If ‘R’ is the location of the gene for Rhesus factor (Rh) antigen production, then locus ‘r’ is an alternate form of that gene. The gene ‘r’ is almost the same as the gene ‘R’ except for a small spelling difference that does not allow the proper production of the Rh antigen, thus red blood cells will be Rh- rather than Rh+.



4. A Dominant Trait is one which is expressed even when an allele for that characteristic is present on only ONE chromosome. It is a heterozygous condition when the alleles are different.

Rr = heterozygous Rh antigen producer


5. A Recessive Trait is one in which BOTH homologous chromosomes

MUST possess the same message in order for that trait to be observed in the person.


It occurs as homozygous condition – two identical alleles.

rr = altered, non-Rh antigen production

A recessive allele (r) can be hidden by the presence of the functional form of the gene (R).


6. Dominant alleles are designated by capital letters (R, A, G);

Non-functional alleles are indicated by lower case letters (r, g,h)



Answer ALL Questions from this point in writing.

Reread the opening description of the Waverly Family.

a. Which trait APPEARS to be dominant in nature? ____________

b. Which trait appears to be recessive in nature? ____________

c. What are the genotypes (the actual allele codes) of the family members?


WRITE A KEY before you attempt to answer this or any question.

Select a letter to represent hair condition. Choose only ONE letter.


KEY Curly Hair Allele = _____

Non-functional Allele = _____


USE THE KEY LETTERS to describe the genotype of each family member.

Member Phenotype Genotype

observable characteristic actual gene code

Grandma Waverly ____________________ _______

Mr. Waverly ____________________ _______

Mrs. Waverly ____________________ _______

Children type 1 ___________________ _______

type 2 ___________________ _______


d. What is the probability of the Waverly’s NEXT child having straight hair?







Punnett square

Homozygous, Heterozygous

Dominant, Recessive

Phenotype, Genotype


A. Free ear lobe formation is dominant over attached ear lobe development.

1. What is the genotype of a person with attached ear lobes?

If you give the gene for earlobe formation the letter E/e, what letter type (capital or lower case) would you use for the attached ear lobe gene?


2. What possible genotypes may a person with free ear lobes have?




3. If two people with free ear lobes have a child with attached ear lobes, what is their genotype? ____________________________________________________________









B. Polydactyly is a condition in which a person possesses extra toes or fingers.

This condition is dominant.

1. What is your genotype for the number of digits? _______

WRITE A KEY BEFORE ANSWERING QUESTION (use P or p for the gene that can cause polydactyly) __________________


2. If you were to marry a person who has polydactyly, what are the possible

phenotypes of your children? _______________________________________






C. A form of dyslexia that may affect one out of every 100 Americans has been linked to an inherited chromosome defect. Based on a study of 16 families with a history of dyslexia, scientists concluded that one out of three inherited cases of the reading disorder is linked to a defect on chromosome 6.


1. If a child MUST inherit TWO chromosomes, both possessing the non-functional gene, to experience dyslexia, how would you describe this condition? Is it inherited in a dominant or recessive fashion?



2. If a woman with this form of dyslexia marries a man carrying one defective gene on chromosome 15, what are the chances that their first child may experience dyslexia? Explain.

______________________________________________________ ______________________________________________________ ______________________________________________________


D. Duchenne Muscular Dystrophy begins to show its presence at an early stage in a child’s life. Muscle tissue is replaced by scar tissue. The wasting away of the muscle is seen in the youngster’s wide gait while walking. The defective gene is located on the ‘p’ arm (top) of the X chromosome.


1. Why is the incidence of Duchenne’s Muscular Dystrophy highest in males?

______________________________________________________ ______________________________________________________ ______________________________________________________


2. Why is this condition rarely transmitted to the child by the father?

______________________________________________________ ______________________________________________________ ______________________________________________________

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