TEAS 2.3 Mendel's Laws

What is inheritance?

The transmission of characteristics from parents to offspring through genes.

What is Mendelian inheritance?

The pattern of inheritance that follows Gregor Mendel's laws, discovered through his 19th-century experiments with pea plants.

What is a gene?

A unit of heredity (what Mendel called "factors").

What is an allele?

A specific variant or copy of a gene.

What is a genotype?

The genetic makeup of an individual (the actual alleles present), represented by letters (e.g., PP, Pp, pp).

What is a phenotype?

The physical appearance or expression of a trait, determined by both genetics and environment.

What does homozygous mean?

Carrying two identical alleles (e.g., AA or aa).

What does heterozygous mean?

Carrying two different alleles (e.g., Aa).

What is the difference between homozygous dominant and homozygous recessive?

Homozygous dominant has two dominant alleles (AA), while homozygous recessive has two recessive alleles (aa).

What is Mendel's Law of Dominance?

When two different alleles are present, the dominant trait is always expressed and masks the recessive allele.

What is a dominant trait?

A trait that is always expressed when present and masks the recessive allele (represented by a capital letter).

What is a recessive trait?

A trait that is only expressed when both alleles are recessive (represented by a lowercase letter).

If a pea plant has genotype Gg (where G = green seeds, g = yellow seeds), what color will the seeds be?

Green, because green is dominant and will be expressed.

What is Mendel's Law of Segregation?

Parental genes must separate equally and randomly into gametes during meiosis, so offspring have an equal chance of inheriting either allele.

How many alleles does each parent contribute for each trait?

One allele for each trait.

What is Mendel's Law of Independent Assortment?

Alleles for different traits are inherited independently of each other; they are not linked together.

How many pairs of chromosomes do humans have?

23 pairs (46 total chromosomes).

Where do offspring get their chromosomes from?

One chromosome from each pair comes from each parent.

What are gametes?

Sex cells (eggs or sperm) that contain half the normal number of chromosomes.

How many chromosomes do human gametes contain?

23 chromosomes (half the normal amount).

What is a Punnett square?

A chart used to predict the probability of offspring inheriting specific genotypes and phenotypes.

In a monohybrid cross of PP × pp, what will the F1 generation look like?

All offspring will be Pp (100% heterozygous with the dominant phenotype).

In a monohybrid cross of Pp × Pp (F2 generation), what is the genotype ratio?

1:2:1 (25% PP, 50% Pp, 25% pp).

In a monohybrid cross of Pp × Pp (F2 generation), what is the phenotype ratio?

3:1 (75% dominant phenotype, 25% recessive phenotype).

What is a dihybrid cross?

A cross that tracks the inheritance of two different traits simultaneously.

What is the phenotype ratio for an F2 dihybrid cross?

9:3:3:1

What does the 9:3:3:1 ratio in a dihybrid cross demonstrate?

Independent Assortment - it shows that traits are inherited independently, not as a package.

What is meiosis?

The process of cell division that produces gametes with half the chromosome number.

What is crossing over in meiosis?

When homologous chromosomes exchange genetic material, creating new combinations of alleles during meiosis I.

How does independent assortment create genetic variation?

Homologous chromosomes randomly separate, so each gamete gets a random mix of maternal and paternal chromosomes.

What is non-Mendelian inheritance?

Inheritance patterns that do not follow simple dominant-recessive relationships.

What are multiple alleles?

When more than two alleles exist for a gene (e.g., blood types with A, B, and O alleles).

What is incomplete dominance?

When heterozygous individuals show an intermediate phenotype (e.g., red + white = pink flowers).

What is co-dominance?

When both alleles are fully expressed simultaneously (e.g., AB blood type expresses both A and B proteins).

What is epistasis?

When one gene affects the expression of another gene through interactions between different genes.

What is sex-linked inheritance?

Genes located on X or Y chromosomes that show different expression patterns in males (XY) versus females (XX).

How can you identify non-Mendelian inheritance?

If the F2 generation does NOT show 3:1 or 9:3:3:1 ratios, indicating factors beyond simple dominant-recessive relationships.

What are inherited traits?

Traits passed from parent to offspring through genes (e.g., eye color, blood type, seed color).

What are non-inherited traits?

Traits not coded in genes, such as culturally influenced behaviors or learned skills.

Why are all gametes genetically unique?

Because of crossing over and independent assortment during meiosis, creating different combinations of traits than their parents.

If both parents are heterozygous for a trait (Aa × Aa), what percentage of offspring will be homozygous recessive?

25% (aa).

In a cross between PP and pp plants, can any offspring be homozygous recessive?

No, all offspring will be Pp (heterozygous).

What letters are used to represent dominant alleles in genetics?

Capital letters (e.g., P, A, G).

What letters are used to represent recessive alleles in genetics?

Lowercase letters (e.g., p, a, g).

What is the P1 generation?

The parental generation (the first set of parents in a genetic cross).

What is the F1 generation?

The first offspring generation resulting from a P1 cross.

What is the F2 generation?

The second offspring generation resulting from crossing F1 individuals.

How many different phenotypes appear in an F2 dihybrid cross?

Four different phenotypes (demonstrating independent assortment).

What does it mean when a trait "masks" another trait?

The dominant trait is expressed and hides the recessive trait when both alleles are present.

Can a heterozygous individual express a recessive phenotype?

No, heterozygous individuals will express the dominant trait.

What must you know to use a Punnett square?

The genotypes of both parents.

In the example of pea plant flower color, which color is dominant: purple or white?

Purple (P) is dominant; white (p) is recessive.