Understanding Inheritance

Understanding inheritance

• Mendel concluded that two factors control each trait.

• He also concluded that they can be dominant or recessive.

• One factor comes from the egg cell and one factor comes from the sperm cell.

• What are these factors? How are they passed from parents to offspring?

What controls traits?

What controls traits? c h r o m o s o m e s

• Inside each cell is a nucleus that has chromosomes. Chromosomes contain genetic information that controls traits.

• What Mendel called “factors” are parts of chromosomes. We know them as genes.

What controls traits? c h r o m o s o m e s

• Each cell in an offspring contains chromosomes from both parents.

• These chromosomes exist in pairs — one chromosome from each parent.

What controls traits? g e n e s & a l l e l e s

• Each chromosome can have information about 100’s or 1000’s of traits.

• A gene is a section on a chromosome that has genetic information for one trait.


• An offspring inherits two genes for each trait, one from each parent.

• The genes can be the same or different, such as purple or white for pea flower color.

XX


• Another example of a trait and its forms is having dimples or not having dimples.


• The different forms of a gene are called alleles.

What controls traits? g e n o t y p e & p h e n o t y p e

• Geneticists call how a trait appears, or is expressed, the trait’s phenotype.

• A person’s eye color is an example of phenotype. The trait of eye color can be expressed as blue, brown, green, or other colors.


• Mendel concluded that there were two alleles that controlled the phenotype of each trait.

• The two alleles that control the phenotype of a trait are called the trait’s genotype.

• You cannot see an organism’s genotype. But you can make guesses about a genotype based on its phenotype.


• Scientists use symbols to represent the alleles in a genotype.

• Uppercase letters represent dominant alleles and lowercase letters represent recessive alleles.

• The dominant allele, if present, is written first.

heterozygous: Rr

What controls traits? homozygous & heterozygous

homozygous:RRrr

modeling inheritance

• Plant breeders and animal breeders use two tools to help them predict how often traits will appear in offspring.

• These tools, Punnett squares and pedigrees, can be used to predict and identify traits among genetically related individuals.

Pedigree


Punnet t Square


• If the genotypes of the parents are known, then the different genotypes and phenotypes of the offspring can be predicted.

• A Punnett square is a model used to predict possible genotypes and phenotypes of offspring.

P u n n e t t S q u a r e s

modeling inheritanceP u n n e t t S q u a r e s


• A pedigree shows phenotypes of genetically related family members.

• It can also help determine genotypes.

P e d i g r e e s


• Let’s look at the phenotypes of the children.....

P e d i g r e e s

complex patterns of inheritance

• Mendel studied traits influenced by only one gene with two alleles.

• We know now that not all traits are inherited this way. Some traits have more complex inheritance patterns.

t y p e s o f d o m i n a n c e :


• Alleles show incomplete dominance when the offspring’s phenotype is a blend of the parents’ phenotypes.

• Here is an example with 4 o’clock flowers.

i n c o m p l e t e d o m i n a n c et y p e s o f d o m i n a n c e :


i n c o m p l e t e d o m i n a n c et y p e s o f d o m i n a n c e :


• When both alleles can be observed in a phenotype, this type of interaction is called codominance.

c o d o m i n a n c et y p e s o f d o m i n a n c e :


• For example, if a cow inherits the allele for white coat color from one parent and the allele for red coat color from the other parent, the cow will have both red and white hairs.

c o d o m i n a n c et y p e s o f d o m i n a n c e :


• Unlike the genes in Mendel’s pea plants, some genes have more than two alleles, or multiple alleles.

• Human ABO blood type is an example of a trait that is determined by multiple alleles.

• There are three alleles for the ABO blood type—IA, IB, and i.

m u l t i p l e a l l e l e s


• In the pea plants there were two possible phenotypes, ie round pea or wrinkled pea.

• The way the alleles combine for blood types, results in one of four blood types—A, B, AB, or O.



• The IA and IB alleles are codominant to each other, but they both are dominant to the i allele.

• Even though there are multiple alleles, a person inherits only two of these alleles, one from each parent.



• Mendel concluded that only one gene determined each trait. We now know that more than one gene can affect a trait.

• Polygenic inheritance occurs when multiple genes determine the phenotype of a trait.

p o l y g e n i c i n h e r i t a n c e


• Because several genes determine a trait, many alleles affect the phenotype even though each gene has only two alleles.

• Therefore, polygenic inheritance has many possible phenotypes.

• Eye color, height, weight and skin color are all examples of traits that are determined by polygenic inheritance.


genes and the environment

• Genes are not the only factors that can affect phenotypes. An organism’s environment can also affect its phenotype.

• For example, the flower color of one type of hydrangea is determined by the soil in which the hydrangea plant grows.

• Acidic soil produces blue flowers. Basic soil produces pink flowers.


• The flower color of one type of hydrangea is determined by the soil in which the hydrangea plant grows.

• Acidic soil produces blue flowers. Basic soil produces pink flowers.


• Himalayan rabbits have alleles for dark-coloured fur, which is only displayed at lower temperatures.

• The fur in areas further from the rabbit’s body heat i.e. nose, ears, feet, are therefore darkly coloured.


• For humans, healthful choices can also affect phenotype.

• Many genes affect a person’s chances of having heart disease.

• However, what a person eats and the amount of exercise he or she gets can influence whether heart disease will develop.

recessive genetic disorders

• Disorders caused by recessive genes. You must have both recessive alleles in order to have the disorder.

X YXX

sex-linked disorders

• Red-green colour blindness is a recessive sex-linked disorder.

• It occurs on the X chromosome.

XXfemalenot color blind

XXfemale

not color blind female

color blind

X Ymale

not color blind male

color blind

sex-linked disorders

Understanding Inheritance

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