How do genes determine inherited traits?

How Genes Determine Inherited Traits

Genes are segments of DNA that carry instructions for building proteins, which perform essential functions in living organisms. These instructions are encoded in the sequence of nucleotide bases (adenine, thymine, cytosine, guanine). Each gene occupies a specific location (locus) on a chromosome.

Inherited traits are characteristics passed from parents to offspring through genes. Each individual inherits two copies of each gene-one from each parent. The combination of gene variants (alleles) determines the trait expressed.

Worked Example: Mendelian Inheritance

Consider a gene with two alleles: $A$ (dominant) and $a$ (recessive). The possible genotypes and their corresponding phenotypes are:

• $AA$: dominant trait expressed
• $Aa$: dominant trait expressed
• $aa$: recessive trait expressed

Suppose both parents are heterozygous ($Aa$). What are the possible genotypes and probabilities for their offspring?

Step 1: Set up the Punnett square

A (parent 2)	a (parent 2)
A (parent 1)	AA	Aa
a (parent 1)	Aa	aa

Step 2: List possible genotypes

• $AA$
• $Aa$
• $Aa$
• $aa$

Step 3: Calculate probabilities

• $AA$: $\frac{1}{4}$
• $Aa$: $\frac{2}{4} = \frac{1}{2}$
• $aa$: $\frac{1}{4}$

Step 4: Determine phenotypes

• Dominant trait ($AA$ or $Aa$): $\frac{3}{4}$
• Recessive trait ($aa$): $\frac{1}{4}$

Takeaways

• Genes encode instructions for traits, and alleles determine trait variations.
• Offspring inherit one allele from each parent, leading to different genotype combinations.
• Dominant alleles mask recessive ones, affecting trait expression according to Mendelian ratios.