Principles of Inheritance and Variation

The father, because he produces X and Y bearing sperms

Step 1: In humans, females are homogametic (XX) — they produce only one type of egg, each carrying one X chromosome. Step 2: Males are heterogametic (XY) — they produce two types of sperms: 50% carry X chromosome and 50% carry Y chromosome. Step 3: If a Y-bearing sperm fertilizes the egg (X), the offspring is XY = Male. Step 4: If an X-bearing sperm fertilizes the egg (X), the offspring is XX = Female. Step 5: Since the mother always contributes an X chromosome, the sex of the child depends entirely on which type of sperm (X or Y) from the father fertilizes the egg. Therefore, the father deter

Testes Determining Factor (TDF)

Step 1: The Y chromosome in humans contains a special region called the Non-combining Region of the Y (NRY), which has functional (euchromatic) and non-functional (heterochromatic) genes. Step 2: Within the euchromatic region of the Y chromosome lies the SRY gene (Sex determining Region Y). Step 3: The SRY gene encodes (produces) a protein called Testes Determining Factor (TDF). Step 4: TDF triggers the development of testes from the undifferentiated gonads in the embryo, initiating male development. Step 5: In the absence of the Y chromosome (and therefore no SRY gene and no TDF), female deve

0.50

Step 1: C.B. Bridges proposed that sex in Drosophila is determined by the ratio of X chromosomes to sets of autosomes (Sex Index = X/A). Step 2: A normal diploid male Drosophila has the chromosomal constitution 2A + XY (1 X chromosome and 2 sets of autosomes). Step 3: Sex Index for normal male = 1 X / 2 A sets = 0.50. Step 4: A normal female has 2A + XX, so Sex Index = 2X / 2A = 1.0. An intersex has Sex Index = 0.67 (2X/3A). A metafemale (superfemale) has 1.5 (3X/2A). Step 5: When the sex index is 0.50 or less, the organism is male; when it is 1.0 or more, the organism is female; 0.67 gives an

The trait passes from father to grandson through carrier daughter

Step 1: Haemophilia is caused by a recessive allele on the X chromosome. Males are hemizygous (X^h Y), meaning they express the disease with just one copy of the recessive allele. Step 2: In a marriage between a colour blind/haemophilic man (X^h Y) and a normal woman (X^H X^H): F1 daughters are X^H X^h (carriers) and F1 sons are X^H Y (normal). Step 3: The affected father does NOT directly pass the trait to his sons (sons get Y from father). Instead, he passes the X^h chromosome to his daughters, who become carriers. Step 4: When these carrier daughters (X^H X^h) marry normal men (X^H Y), 50%