Heredity

The chromosome inherited from the father

Step 1: Women have XX sex chromosomes, while men have XY sex chromosomes. Step 2: All eggs from the mother contain an X chromosome. Step 3: Sperm from the father can contain either an X or a Y chromosome. Step 4: If the sperm carries an X chromosome, the child will be XX (female). Step 5: If the sperm carries a Y chromosome, the child will be XY (male). Therefore, the father's chromosome determines the child's sex.

23 pairs

Step 1: Humans have a total of 46 chromosomes in each cell. Step 2: These 46 chromosomes are arranged in pairs, with one chromosome from each parent. Step 3: 46 ÷ 2 = 23 pairs of chromosomes. Step 4: Out of these 23 pairs, 22 pairs are autosomes (non-sex chromosomes) that are perfectly matched. Step 5: The 23rd pair consists of sex chromosomes (XX in females, XY in males) which may not be perfectly matched in males.

A trait that is expressed only when two copies are present

Step 1: A recessive trait requires both copies of the gene to be the same for the trait to be expressed. Step 2: Using Mendel's example, shortness (t) is recessive to tallness (T). Step 3: For a plant to be short, it must have 'tt' - both copies must be for shortness. Step 4: If even one copy is 'T' (as in 'Tt'), the plant will be tall because the dominant trait masks the recessive one. Step 5: This is why recessive traits seem to 'skip' generations - they can be carried hidden and only appear when both parents contribute the recessive gene.

New combinations of traits appear in F2

Step 1: The F1 plants were tall with round seeds, carrying genes for both tall/short and round/wrinkled. Step 2: During reproduction, genes for different traits can separate and recombine independently. Step 3: This independent assortment creates new combinations not seen in the parents. Step 4: F2 generation shows: tall-round, tall-wrinkled, short-round, and short-wrinkled plants. Step 5: The appearance of tall-wrinkled and short-round plants (new combinations) proves that traits are inherited independently, which is Mendel's law of independent assortment.