Lab 6 - Inheritance of Traits: the Dragon Genetics Lab

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Students will be able to:

• Define the terms heredity, traits, karyotype, autosome, Punnett square, alleles

• Determine the difference between dominant and recessive genes.

• Compare homozygous and heterozygous traits.

• Demonstrate how alleles affect the inheritance of traits in a model organism.

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• The passing of characteristics from parent to offspring is called heredity.

• Before DNA and chromosomes were discovered, the mechanism of heredity was one of the greatest mysteries of science.

• It was once believed (in the 16th century) that a sperm contained a fully formed, miniature human being (called preformationism).

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Terminology to know

• DNA - deoxyribonucleic acid, a self-replicating material present in nearly all living organisms as the main constituent of chromosomes. It is the carrier of genetic information.

• Chromosomes - a threadlike structure of nucleic acids and protein found in the nucleus of most living cells, carrying genetic information in the form of genes.

• Genes - a distinct sequence of nucleotides forming part of a chromosome, and determines some characteristics of offspring

• Allele - one of two or more alternative forms of a gene

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• Autosomal Chromosome – pairs 1 – 22 in humans, contain all the instructions for the body’s anatomy and function

• Sex Chromosome – 23rd pair in humans, contain the sex instructions, xx or xy

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Gregor Mendel

• An Austrian monk named Gregor Mendel, who is known as the Father of Genetics, pioneered the study of heredity in the late 1800’s.

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Mendel’s Hypotheses

• For each inherited character, an individual has two copies of the gene -one from each parent.

• These are called alleles

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Mendel’s Hypotheses

1. There are alternative versions of the gene - for example, the alleles (big) Y and (little) y, (big) T and (little) t– The combination of

these alleles is the organism’s genotype for that gene

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Mendel’s Hypotheses

• Of the two alleles, one of them may be completely expressed (dominant), while the other one may have no observable effect (recessive) on the organism’s phenotype (appearance).

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Mendel’s Hypotheses

• When gametes are formed, the alleles for each gene in an individual separate independently of one another. Thus gametes carry only one allele for each inherited trait.

• Gametes are the sex cells – the egg or sperm

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Two Laws of Heredity

• The law of segregation states that the two alleles for a character segregate (separate) when the gametes are formed.

• The law of independent assortment states that the alleles of different genes separate independently of one another during gamete formation. For example, the alleles for the height of a plant separate independently of the alleles for its flower color.

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The Human Karyotype

22 pairs of autosomes

1 pair of sex chromosomes

Female = XXMale = XY

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For autosomal genes:

• If an individual’s genotype is homozygous dominant or heterozygous, their phenotype will show the dominant allele. Ie. HH, Hh

• If an individual’s genotype is homozygous recessive, their phenotype will show the recessive allele. Ie. hh

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Autosomal or Sex Linked?• If a gene is autosomal (found on the autosomes), it

will appear in both sexes equally.• A sex-linked gene’s allele is located on the X or Y

(sex) chromosome.• Most sex-linked traits are carried on the X

chromosome and are recessive.• If a trait is sex-linked, its effects are usually seen

only in males.– Color-blindness– Male-pattern baldness– Hemophilia

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Sex-Influenced Traits• A sex-influenced trait is a phenotype that is expressed differently

in males and females.• While sex-linked genes are only found on the sex chromosomes,

sex-influenced genes can be autosomal• Example:– Male pattern baldness (due to high testosterone; males have

more testosterone than females, so baldness is much more common in males)

• Also called a sex-limited trait (trait is found in only one sex)

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Complex Patterns of Heredity

• Not all traits that are inherited are controlled by one set of genes.

• When several genes influence a character, this is called polygenic inheritance– eye color, weight, height, skin color (this is why we have so

many different eye colors, weights, skin colors, etc)

• In Mendel’s crosses there was either a dominant or a recessive trait. In some organisms, an individual displays a phenotype that is intermediate between the two parents, a condition known as incomplete dominance

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Examples of Incomplete Dominance

• When a snapdragon with red flowers is crossed with a snapdragon with white flowers, a snapdragon with pink flowers is produced. Neither the red nor the white allele is dominant over the other.

• In humans, the child of a straight haired parent and a curly-haired parent will have wavy hair.

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Incomplete Dominance v. Codominance

• Example: flower color in Carnations– Homozygous dominant (AA) = RED phenotype– Homozygous recessive (aa) = – Heterozygous (Aa) = PINK

• Codominance: both alleles are equally dominant and both alleles are visible in the hybrid genotype

• Example: feather color in chickens– Homozygote 1 (F1F1) = BLACK feathers

– Homozygote 2 (F2F2) =

– Heterozygote (F1F2) = Black and

pattern

Incomplete dominance: one allele is only partially dominant to the other; the heterozygote exhibits a phenotype that is intermediate relative to the dominant and recessive phenotypes

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Characters Influenced by the Environment

• Some phenotypes depend on environmental conditions• Hydrangea flowers of the same genotype can exhibit

different phenotypes based on the environment (soil pH) • Plants in acidic soil have blue flowers, while

plants in neutral or basic soil have pink flowers

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Temperature-dependent Sex Determination

• This is the condition where the sex of the offspring is influenced by the prevailing temperatures during embryonic development. In sea turtles, warmer temperatures produce more or all females, cool temperatures produce more or all males, and the pivotal temperature is the constant incubation temperature that produces equal numbers of males and females.

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Example from lab

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F f Long neck

G G No back hump

h h Back spikes

i i Short tail

J J Flat feet