Welcome Genetic Counselors in Training

The Case of The Pink Nose Welcome Genetic Counselors in Training

A young Mr. and Mrs. Potato Head are ready to have their first little spud, a baby tater tot. As excited as they are to have a new bundle of joy to care for, they are extremely worried about one thing. What will tater tot’s nose look like? Mr. PH’s nose is most appealing. Unfortunately, Mrs. PH’s pink nose, in their opinion is rather unflattering. It would be their little spud’s best interest to have the nose of Mr. PH. But, what are the chances? That is why they have come to you. They want you to give them as much information as possible so they can make the right decision.

Back to the basics….. • Genetics – the study of inherited traits • Traits are passed down from one generation to the next when genes are passed from parent to offspring. • Traits are passed on structures called chromosomes • Consists of 40% DNA and 60% protein

Sections of DNA code for genes. • Each chromosome carries hundreds of genes • Humans have 46 chromosomes • 22 pairs of autosomes • 1 pair of sex chromosomes • Female has two X chromosomes • Male has one X and one Y chromosome

Karyotype • A karyotype is used to show similar chromosomes paired up next to each other based on size and shape. • The similar pairs of chromosomes are called homologous chromosomes, or homologs. 23rd pair of chromosomes, the sex chromosomes, identify an individual as male or female

Homologous chromosomes carry the same genes, but may contain different versions of the same gene. • Different versions of the same gene are known as ALLELES.

Meiosis Organisms pass their genes from parent to offspring via the cells that are produced from a type of cell division called meiosis. In humans, meiosis occurs in the ovaries in females and the testis in male, producing eggs and sperm. Cells produced in meiosis are called gametes and contain ½ the number of chromosomes and genes. When a gamete from a female fuses with a gamete from a male, genetic information from each partner is passed (and present) in the offspring.

Dominant and Recessive Alleles • Some genes that are passed from the parents are dominant, or expressed, when one allele is present. • Some alleles are recessive and will only be expressed of the same allele is passed down from both parents

Back to Mr and Mrs PH • What do we know? • In the world of potato heads • Dominant = red nose (R) • Recessive = pink nose (r) • Mrs. PH has a pink nose • Received the recessive trait from both parents • Genotype = rr • Phenotype = pink nose • She is homozygous recessive for nose color “homo” – same “zygous” – join together

What do we know about Mr PH? • Phenotype – red nose • So what did he receive from his parents? Possibility #1 • Dominant trait from both parents • Genotype = (RR) • He may be homozygous dominant “homo” – same “zygous” – join together OR…

Possibility #2 • Mr PH received the dominant form of the trait from one parent and the recessive trait from the other parent. • Phenotype – red nose • Genotype – Rr • Mr PH may be heterozygous for nose color. “hetero” – different “zygous “- join together What does this mean?

Mrs PH will pass down (r) and there is a chance, if Mr PH is heterozygous for nose color, he could pass down the recessive form of the trait (r) or the dominant form of the trait (R). Or • Mrs PH will pass down (r) and, if Mr PH is homozygous for nose color, he could pass down just the (R) form of the trait. As long as the dominant trait is passed down by one parent, the dominant allele will be the phenotype

Warm up # ______ • What is Mrs PH’s phenotype and gentoype? • How do you know what her genotype and phentype is? • Answer the same two questions for Mr PH. Be ready to share your answer!

Punnett Square • Used to determine the probability of tater tot having a pink nose Step 1 • What do we know? • Red nose = dominant • Pink nose = recessive

Step 2 • Substitute the trait, nose color, with a letter. We can use any letter, but oftentimes, we use the first letter of the dominant trait, which would be R (for red) Red nose – dominant – R Pink nose – recessive - r

Step 3 • What are we crossing? • We are crossing the genotypes of Mr and Mrs PH. rr X RR Mr. PH was happy to inform us his parents were both homozygous dominant (RR) for nose color! Mrs PH Mr PH as homozygous Possibility #1

Step 4 • Construct a punnett square with Mr and Mrs PH’s genotypes r r R R

What’s in the box? Combine the genotypes (a) (b) r r R R (c) (d) Recommend to put the dominant allele 1st

Step 5 This box represents all of the possible allele combinations Tater Tot may receive from Mr and Mrs PH. Do you see all boxes are the same? There is a 4 out of 4 (4:4, 4/4, 100% )probability that: Genotype = Rr heterozygous for nose color Phenotype = Red nose

Welcome Genetic Counselors in Training