Genetics And Evolution

Step 1: Understanding the Concept:
Genes are the fundamental physical and functional units of heredity.

Step 2: Detailed Explanation:
A gene is a specific sequence of nucleotides in DNA that encodes the synthesis of a gene product, either RNA or protein.
DNA is tightly coiled and packaged into structures called chromosomes. Therefore, genes are arranged linearly along the length of chromosomes, which reside in the nucleus of eukaryotic cells.
The specific physical location of a gene on a chromosome is called its locus.

Step 3: Final Answer:
Genes are located on chromosomes.

Step 1: Understanding the Concept:
When organisms die, they usually decompose completely. However, under specific geological conditions, their remains can be preserved.

Step 2: Detailed Explanation:
Fossils are the naturally preserved physical evidence of past life, typically found embedded in sedimentary rocks.
They can include hard parts like bones, teeth, and shells, or even impressions like footprints or leaf prints.

Step 3: Final Answer:
A fossil is any preserved evidence of life from a past geological age.

Step 1: Understanding the Concept:
Fossils act as historical records of biological history on Earth.

Step 2: Detailed Explanation:
Evidence of Evolution: Fossils show that organisms have changed over time, proving that earlier, simpler forms gradually evolved into modern, complex forms.
Connecting Links: Certain fossils, like Archaeopteryx, possess characteristics of two different groups (reptiles and birds), acting as "missing links" and proving common ancestry.
Geological Time: Index fossils help geologists determine the relative age of rock layers.

Step 3: Final Answer:
Fossils are important because they are the most direct evidence of evolutionary history and past biodiversity.

Step 1: Understanding the Concept:
A dihybrid cross is a breeding experiment between two organisms that are identically hybrid for two traits.

Step 2: Detailed Explanation:
According to Mendel's Law of Independent Assortment, the alleles for two different traits assort independently into gametes.
When considering two traits with complete dominance (e.g., seed shape and seed color), crossing two heterozygous individuals (AaBb AaBb) produces a specific distribution of observable traits (phenotypes) in the F2 generation.
The resulting phenotypic ratio is:
9 (Dominant for both traits) : 3 (Dominant for first, Recessive for second) : 3 (Recessive for first, Dominant for second) : 1 (Recessive for both traits).

Step 3: Final Answer:
The phenotypic ratio is 9:3:3:1.

Step 1: Understanding the Concept:
Sex determination in humans is governed by the XY chromosome system.
Females have two X chromosomes (XX) and males have one X and one Y chromosome (XY).

Step 2: Detailed Explanation:
A human egg (ovum) always carries an 'X' chromosome because females can only pass on an 'X'.
A human sperm can carry either an 'X' or a 'Y' chromosome.
If an 'X' bearing sperm fertilizes the 'X' bearing egg, the resulting zygote will have an 'XX' genetic constitution.
Individuals with an 'XX' genotype develop into biological females.

Step 3: Final Answer:
The sex of the child will be female.