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Mendelian genetics and punnett squares





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Mendelian genetics and punnett squares


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High Yield Notes
11 pages

Mendelian genetics and punnett squares

6 flashcards

USMLE® Step 1 style questions USMLE

1 questions

A 21-year-old woman presents to her outpatient provider for prenatal counseling. Both the patient and her partner are carriers of cystic fibrosis. What is the probability that the couple's child will be a carrier for cystic fibrosis?  


Genetics is the science that studies inheritance, or the way parents transmit certain traits to their descendants.

And Mendelian genetics, refers to Gregor Mendel—an Austrian monk—who studied inheritance by experimenting on pea plants.

He cross-pollinated the flowers of different plants together, took the seeds the developed from the pairing, planted those seeds, and took careful notes on the types of peas that resulted in the subsequent generations. As a monk he was just trying to find his inner peas (peace)!

Now in addition to having lots and lots of peas in his garden, he helped to formulate two important laws; the law of segregation and the law of independent assortment.

So to start out - Mendel took plants with violet flowers and plants with white flowers and crossbreed them.

This original group of flowers are called the P generation, as in “parent,” and then when he obtained some peas, he planted them and got more plants and the flowers in this offspring generation was called F1, or filial one.

It turned out that the F1 generation consisted of all violet flowers, so he called the violet trait “dominant,” while the white trait which appeared to be lost in the F1 generation, was called “recessive.”

Next, Mendel let the violet flowers in the F1 generation cross-pollinate amongst themselves, and when they formed peas - he planted them again.

He got more plants and the flowers from that second generation of plants he called filial two or F2.

It turned out that some of the plants in this F2 generation had white flowers whereas other plants had purple flowers! In fact, the ratio was about 3 violet flowering plants for every 1 white flowering plant.

Based on this experiment, Mendel drew a few conclusions.

First, since the F1 violet flowers had some offspring plants that produced violet flowers and other offspring plants that produced white flowers, it meant that the F1 plants must have contained both of these elements.

The inheritable elements of pea plants are its the gametes, so that meant that the gametes of the F1 plant, contained either the dominant violet trait or the recessive white trait.

The F2 plants are created with one gamete from each parent.

And Mendel worked out that the white flowering plants resulted when they receive both white flower elements, and that plants that had at least one violet flower element from either parent would produce violet flowers.

Mendel didn’t know this at the time, but the “element” he was referring to were segments of DNA called genes that encoded flower color.

These genes were located on specific parts of chromosomes, called loci.

Different versions of a gene are called alleles, and in the case of the flowers there were two alleles - a white and violet allele for flower color.

A helpful way to visualize Mendel’s experiment is to use a Punnett square.

Imagine a box with four squares within in, were we put the genetic information of one parent—or genotype—on the horizontal row and the other parent on the vertical column.

The dominant allele, represented with a capital letter, codes for a violet flower; and the recessive allele, represented with a lowercase letter, codes for a white flower.

The letter we choose doesn’t matter, so let’s use capital P for the violet flower allele, and a lowercase p for the white flower allele.

In the P generation Mendel used pure-breeding plants, so their genotype was two of the same alleles.

In other words, both of the parent plants in this generation were homozygous for flower color trait.

“Homo” meaning same, and zygous referring to the male and female alleles.

The violet pea plant had two of the same dominant alleles “PP”, and therefore had all violet flowers, whereas the white pea plant had two of the same recessive alleles “pp”, and therefore had all white flowers.


Mendelian genetics is the study of how genes are passed from parents to their offspring. Genes are inherited in pairs, one gene from each parent. Punnett squares are a tool used by geneticists to predict the possible combinations of genes that could be inherited from a particular mating. To use a Punnett square, you first need to determine the genotypes of each parent. Then you can use the Punnett square to predict the possible genotypes of their offspring.