Traits, Heredity + Genes

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Introduction to Heredity + Genetics

Heredity + Chromosomes

Heredity is a term used to refer to the process of parents passing down their genetic information to their offspring. This genetic information is contained in chromosomes. Recall from last week that chromosomes are found in the nucleus of the cell.

SPIRAL: Through sexual reproduction - specifically fertilization in which a sperm cell fuses with an egg cell - each parent contributes 50% of its genes to the offspring - allowing for greater genetic variation within a species.

Dominant or Recessive Traits? Which one will show?

People have noticed for thousands of years that family resemblances were inherited from generation to generation so characteristics that are inherited are called traits. A dominant trait is an observable trait of an organism that masks the recessive form of a trait. A recessive trait of an organism can be masked (meaning it will not show up) by the dominant form of a trait. Two organisms may possibly look alike but have different underlying gene combinations.

So in summary - a dominant trait will show up if at least 1 dominant allele has been inherited; a recessive trait will only show up if 2 recessive alleles have been inherited.

Genotype vs. Phenotype

The way an organism looks and behaves makes up its phenotype. How can you remember this? Just think the 'ph-' in phenotype is like the 'ph' in physical! The phenotype of a tall plant is tall, regardless of the genes it contains. The color of your hair, eyes, if your ear lobes are attached or detached, color of your skin, and shape of your face are all examples of an organism's phenotype.

The gene combination an organism contains is known as it genotype. You cannot always know an organism’s genotype simply by looking at its phenotype. Typically, genotypes are represented with letters, for example Dd, rr, MM, etc.

Why are some letters in the genotype written in uppercase/capital letters and others written in lowercase? This is to show which gene is dominant and which gene is recessive. The dominant gene will always be written with an uppercase letter (i.e. D, M, P, Q, R, etc.) and the recessive gene will always be written with a lowercase letter (i.e. d, m, p, q, r, etc.).

Homozygous vs. Heterozygous + Dominant vs. Recessive

When we discuss genotypes, you may here phrases such as "heterozygous dominant" "homozygous dominant" or even "homozygous recessive". Are you confused? Don't be.

Homozygous refers to both alleles of a gene being the same (both dominant or both recessive). Heterozygous refers to the alleles of a gene being different (one dominant, one recessive).

First, just think of your prefixes ( homo = same, hetero = different) and then look at whether the trait/allele is dominant (CAPITAL LETTER) or recessive (lowercase letter).

Let's practice some by classifying the examples on your page.

Pedigree vs. Punnett Squares

A pedigree is a diagram of family relationships that uses symbols to represent people and lines to represent genetic relationships. These diagrams make it easier to visualize relationships within families. Pedigrees are often used to determine the mode of inheritance (dominate, recessive, etc.) of genetic diseases.
A Punnett Square is a chart wherein all possible gene combinations are shown in a cross of parents. Reginald Punnett was an English Geneticist who discovered some very basic principles of genetics including the determination of sex and linkage. His work involved using the color characteristics of a chicken’s feathers, efficiently separating the female gender of the chicken from the male. In plants, a tall plant is considered dominant over a short plant. In using Punnett Squares, you can easily predict genotype and phenotypes of any offspring.

Check out the video below to learn how to solve Punnett squares!

Learn Biology: How to Draw a Punnett Square