Biology: Animals and Organisms

Heredity and Variation

• Heredity is the transmission of genetically based characteristics from parents to offspring.
• Variation refers to the differences among individuals of the same species.
• Like begets like, meaning that cats produce cats, mango seeds germinate into mango trees, and humans give birth to humans.

Characteristics and Traits

• A character is a feature of an organism, while a trait is a variant of a character.
• Examples of characters and traits include:
  • Eye color (brown or blue)
  • Hair shape (curly or straight)
  • Eyebrows (heavy, bushy, or thin)
  • Hair on the middle joint of fingers (growth or no growth)
  • Color vision (normal or red-green color blindness)
  • Tongue rolling (U-shape or no rolling)
  • Hand use (right-handedness or left-handedness)
  • Skin color (albinism or normal pigment)
  • Ear lobe (free or attached)
  • Lips (thick or thin)
  • Rh blood group (Rh positive or Rh negative)

Chromosomes

• Chromosomes are the carriers of heredity.
• Humans have 46 chromosomes arranged in 23 pairs.
• Chromosomes occur in even numbers and are always found in pairs.
• Homologous chromosomes are pairs of chromosomes with the same shape and size, one inherited from each parent.
• Autosomes are chromosomes that determine general body features, while sex chromosomes determine the sex of an organism.

Karyotype

• A karyotype is a chart that shows the arrangement of chromosomes in a cell nucleus.
• Human karyotype shows 23 pairs of chromosomes.
• Chromosomes are visible only during cell division.

Autosomes and Sex Chromosomes

• Autosomes are chromosomes that determine general body features, such as complexion, height, and seed color.

• Humans have 22 pairs of autosomes.

• Sex chromosomes (also called allosomes) determine the sex of an organism.

• Humans have only one pair of sex chromosomes (XY in males and XX in females).### Sex Determination

• The sex of a child is determined by the type of sperm that fertilizes the egg, not by the mother's ability to produce only daughters.

• Eggs contain only X chromosomes, while half of the sperms released are X-bearing and the other half are Y-bearing.

• When an X-bearing sperm fertilizes an egg, the resulting combination is XX, i.e., a female constitution.

• When a Y-bearing sperm fertilizes an egg, the resulting combination is XY, i.e., a male constitution.

Genes and Genome

• Genes are specific parts of a chromosome that determine hereditary characteristics.
• The genome is the full complement of DNA, including all genes and intergenic regions, of an organism.
• Humans have nearly 19,000 genes, with chromosome 1 having the largest number (2968) and chromosome Y having the fewest (231).

Genes and Their Alleles

• Genes have two alternative forms, called alleles, which produce different effects.
• Dominant alleles express themselves regardless of the presence of another allele.
• Recessive alleles are suppressed in the presence of a dominant allele.
• Example: The gene for tongue rolling has two alleles, one for rolling and one for non-rolling.

Genotype and Phenotype

• Genotype refers to the genetic constitution of an organism, i.e., the kinds of genes it possesses.
• Phenotype refers to the expressed characteristics of an organism, which are genetically controlled.
• Example: For the phenotype of tongue rolling, there are two genotypes: RR (homozygous dominant) and Rr (heterozygous dominant).

Inheritance

• The gene for tongue rolling is located on a particular pair of chromosomes.
• Every individual has a pair of this gene, which can be represented as RR, Rr, or rr.
• The genotype of an individual determines the phenotype, which is the observable characteristic.### Punnett Square and Inheritance
• A Punnett square is a simple diagram used to illustrate the possible combinations of gametes (sex cells) from two parents.
• The square is divided into sub-squares, with the gametes from one parent along one side and the gametes from the other parent along the other side.
• The resulting genotypes and phenotypes can be written in the sub-squares.

Tongue Rolling Inheritance

• The ability to roll the tongue is an inherited trait.
• A pedigree (family) chart can be used to show the inheritance of this trait.
• In the pedigree chart, males are represented by squares and females are represented by circles.
• Both parents are tongue rollers (hollow symbols), but their children can have different genotypes (RR, Rr, or rr).

Dominant and Recessive Traits

• A dominant trait is one that is expressed when an individual has one or two copies of the dominant gene.
• A recessive trait is one that is expressed only when an individual has two copies of the recessive gene.
• Examples of dominant and recessive traits include:
  • Eye color: brown (dominant) and blue (recessive)
  • Tongue rolling: rolling (dominant) and non-rolling (recessive)
  • Hand clasping: right-handed (dominant) and left-handed (recessive)
  • Ear lobe attachment: free (dominant) and attached (recessive)

Heterozygous and Homozygous

• An individual who is heterozygous for a trait has two different forms of the gene (e.g. Rr).
• An individual who is homozygous for a trait has two identical forms of the gene (e.g. RR or rr).

Inheritance of Ear Lobe Attachment

• The inheritance of ear lobe attachment can be explained using a pedigree chart.
• In family A, both parents have free ear lobes (dominant), and their children can have free or attached ear lobes.
• In family B, both parents have attached ear lobes (recessive), and their children all have attached ear lobes.

Polydactyly

• Polydactyly is a rare genetic trait that results in extra fingers or toes.
• The trait is dominant, meaning that an individual only needs to inherit one copy of the gene to express the trait.
• An extreme case of polydactyly is shown in the example of a 15-month-old boy with 7 fingers on each hand and 10 toes on each foot.

Sex-Linked Inheritance

• Sex-linked inheritance refers to the inheritance of traits that are linked to the X chromosome.
• The X chromosome carries genes that are responsible for certain traits, such as color vision.
• Inheritance of these traits follows a specific pattern, as shown in the example of a color-blind father and a normal mother.