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Multiple Alleles (ABO Blood Types) and Punnett Squares

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The mother's genetic code is XN, Xn and the father has XN, XN. Therefore they have a 25% chance of having a child that is a hemophiliac which was their son.

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2.  In humans the gene from normal blood clotting, N, is dominate to the gene for hemophilia, n.  This is a sex-linked trait found on the X chromosome. A woman with normal blood clotting has four children. They are a normal son, a hemophiliac son, and two normal daughters. The father has normal blood clotting. What is the probable genotype for each member of the family?

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(a) Genotype (b)  Genotype

RR - red eyes RR - red eyes

Rr - red eyes Rr - red eyes

rr - white eyes rr - white eyes

Phenotype Phenotype

2 girls will have red eyes and 2 boys will 2 girls will have red eyes and 2 boys

have red eyes will have white eyes

(c) Genotype (d)  Genotype

RR - red eyes RR - red eyes

Rr - red eyes Rr - red eyes

rr - white eyes rr - white eyes

Phenotype Phenotype

2 girls will have red eyes and 2 boys will 2 girls will have red eyes and 2 boys

have red eyes will have red eyes

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1.  In Drosophila, the gene for red eyes, R is dominant for the gene for white eyes, r.  This is sex-linked. Determine the possible genotype and phenotype ratios expected from a cross between, (a) heterozygous female and red-eyed male, (b) a heterozygous female and a white-eyed male, (c) a homozygous dominant female and a red-eyed male, and (d) a homozygous dominant female with a white-eyed male.

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Tay-Sachs disease (the most severe form of Hexosaminidase A deficiency) is a progressive, fatal genetic condition that affects the nerve cells in the brain. People with Tay-Sachs lack a specific protein (enzyme) called hexosaminidase A. This enzyme deficiency causes a fatty substance, GM2 ganglioside, to build up in the brain. It is this accumulation that causes the symptoms of Tay-Sachs.

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A defective gene on chromosome 15 (HEX-A) causes Tay-Sachs disease. This defective gene causes the body to not make a protein called hexosaminidase A. Without this protein, chemicals called gangliosides build up in nerve cells in the brain, destroying brain cells.

The disease is hereditary, which means it is passed down through families. You have to receive two copies of the defective gene — one from each parent — to become affected. If only one parent passes down the defective gene, the child becomes a carrier. They will not be affected, but may pass the disease down to their own children.

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Medication. To reduce your child's symptoms, a number of prescription medications are available, including seizure medications.

Respiratory care. Children who have Tay-Sachs disease are at high risk of lung infections that cause breathing problems and frequently accumulate mucus in their lungs.

Respiratory therapists at Mayo Clinic have extensive experience in reducing the mucus using chest physiotherapy (CPT) and in training family members to provide CPT to your child at home.

Feeding tubes. Your child may develop respiratory problems by inhaling food or liquid into the lungs while eating.

To prevent those problems, your doctor may recommend an assistive feeding device such as a nasogastric tube, which is inserted through your child's nose and goes to your child's stomach. Or, a doctor trained in stomach surgery may surgically insert an esophagogastrostomy tube.

Physical therapy. As the disease progresses, your child may benefit from physical therapy to help keep joints flexible and maintain as much ability to move (range of motion) as possible.

Physical therapy can delay joint stiffness and reduce or delay the loss of function and pain that can result from shortened muscles.

Family support. Your child's Mayo Clinic treatment team can provide resources and information to help you and your family cope with your child's needs. Your team can help connect you to local support groups.

Follow-up care and coordination. Your Mayo Clinic doctor will provide treatment and follow-up care for your child and will coordinate the treatment and support plan with your child's primary care doctor or pediatrician.

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Symptoms of Tay-Sachs usually develop around 3-6 months of age when the child starts to have muscle weakness, low muscle tone, an increased startle response and sudden contractions of large muscles when falling asleep (myoclonic jerks).

Between 6 and 10 months of age, a child will not meet motor milestones and may lose the ability to perform tasks (such as sitting) that he/she had previously learned. Decreased eye movement and contact as well as attentiveness are also seen along with a specific change in the eye called a cherry-red spot which can be seen during an eye exam.

After 8 to 10 months of age, a baby will move less and become less responsive. Vision will be lost and many will have seizures by a year of age. A person's head size will start to grow around 18 months of age and when a child is 2 years old, they typically have trouble swallowing and progress into an unresponsive vegetative state. Age of death is usually between 2 and 4 years, often from pneumonia.

In addition to classic Tay-Sachs, there are other forms of Hexosaminidase A deficiency that are sometimes referred to as forms of Tay-Sachs.

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The classic infantile form is usually fatal by age 2 or 3 years. Death usually occurs due to intercurrent infection.

In the juvenile form, death usually occurs by age 10-15 years; preceded by several years of vegetative state with decerebrate rigidity. Death is usually due to infection.

In the adult form, most patients have a normal life expectancy.