**Question:** A person showing two cell lines derived from one zygote is

A. Heterozygous
B. Homozygous
C. Diploid
D. Haploid

**Correct Answer:** B. Homozygous

**Core Concept:** Zygote is the fertilized egg, formed when a sperm cell fuses with an egg cell. In humans, a zygote undergoes rapid cell division, leading to the formation of a multitude of cells that eventually give rise to an organism. The genetic makeup of the zygote determines the characteristics of the offspring.

**Why the Correct Answer is Right:** A person showing two cell lines derived from one zygote indicates that the individual possesses two identical cell lines. In genetics, this refers to homozygous (homo = same; zeugma = joining) individuals, who have two identical alleles (gene variants) for a particular trait. This can happen due to autosomal recessive inheritance (both parents carry the trait), autosomal dominant inheritance (both parents transmit the trait), or an X-linked recessive trait (both parents are carriers).

**Why Each Wrong Option is Incorrect:**

A. Heterozygous (hetero = different) refers to individuals with two different alleles for a particular trait. This is typically seen in autosomal dominant inheritance or an X-linked dominant trait.

B. Homozygous refers to individuals with two identical alleles for a particular trait, as explained above.

C. Diploid cells contain two sets of chromosomes, one from each parent. Homozygous individuals would have two identical sets of chromosomes, which is not the case here.

D. Haploid cells contain only one set of chromosomes, one from the mother and one from the father. Homozygous individuals possess two identical sets of chromosomes.

**Clinical Pearl:** Understanding the concept of homozygosity is crucial in genetic counseling, especially when assessing carrier status for genetic disorders or predicting offspring's genotype. This knowledge allows healthcare providers to guide families regarding potential risks and inheritance patterns.