1 of 8© Boardworks Ltd 2007 Inheritance terms. Objectives Define key inheritance terms. Determine genotypes and phenotypes of the offspring of a monohybrid

1 of 8 © Boardworks Ltd 2007

Inheritance terms

Objectives

Define key inheritance terms.

Determine genotypes and phenotypes of the offspring of a monohybrid cross using

a punnet grid.

Outcomes

3: Explain how the inheritance of characteristics is controlled

by dominant and recessive alleles.

5: Construct genetic cross diagrams.

7: Predict and/or explain the outcomes of crosses between individuals for each possible combination of dominant and recessive alleles of the same

gene.Key terms: monohybrid, heterozygous, homozygous, dominant,

recessive


Family resemblance

Members of the same family often look similar.

If the son and daughter have children of their own one day, will they also look like their parents?

Which parent do these children look more like?

Humans, like all organisms, inherit characteristics from their parents. How are characteristics passed on?

Why do members of the same family look similar?


Different versions of genes

Chromosomes in a homologous pair contain the same type of genes that code for the same characteristics, such as eye colour.

Each different version of a gene is called an allele.

Each chromosome in the pair, however, may have a different version of the gene.

For example, the version ofa gene on one chromosomemay code for brown eyes,whereas the version of the gene on the other chromosome may code for blue eyes.

allele for browneyes

allele for blue

eyes


Homozygous alleles

If the alleles for a characteristic in a homologous pair are the same, the organism is said to be homozygous for that characteristic.

What colour eyes will these homozygous pairs of alleles produce?

allele forbrown eyes


allele forblue eyes

allele forblue eyes


Heterozygous alleles

The characteristic expressed by heterozygous alleles will depend on which allele is dominant and which allele is recessive.

If the alleles for a characteristic in a homologous pair are different, the organism is said to be heterozygous for that characteristic.

What colour eyes will this heterozygous pair of alleles produce?


allele forblue eyes

?


Dominant or recessive?

Dominant alleles are always expressed in a cell’s phenotype. Only one copy of the dominant allele needs to be inherited in order for it to be expressed. Dominant alleles (e.g. brown eyes) are represented by an upper case letter (e.g. ‘B’).

The phenotype for a particular characteristic depends on which allele is dominant and which allele is recessive.

Recessive alleles are only expressed in a cell’s phenotype if two copies of it are present. If only one copy is present, its effect is ‘masked’ by the dominant allele. Recessive alleles (e.g. blue eyes) are represented by a lower case letter (e.g. ‘b’).


What eye colour?

The allele for brown eyes is dominant over the allele for blue eyes.

The individual will have brown eyes, because the allele for brown eyes masks the allele for blue eyes.


allele forblue eyes

So, what colour will the eyes be of an individual who is heterozygous for eye colour?


Homozygous cross


Heterozygous cross


Finding the genotype

For some characteristics, the genotype of a homozygous recessive individual can be determined from their phenotype.

A test cross can be used to determine whether an individual is homozygous or heterozygous for a dominant trait.

But what about individuals that have brown fur? Is their genotype BB or Bw ?

For example, the allele for brown fur (B) in mice is dominant over the allele for white fur (w). This means that all white mice must therefore have the genotype .


What is a test cross?

During a test cross, an individual with an unknown genotype is crossed with a homozygous recessive individual. The phenotype of the offspring will reveal the unknown genotype.

If all the offspring display the dominant phenotype, then the parent of unknown genotype must be homozygous for the characteristic.

If half the offspring show the dominant phenotype, and half show the recessive phenotype, then the parent must be heterozygous for the characteristic.


Using test crosses to find genotype


For example, when a red Mirabilis jalapa plant (also called the snapdragon or ‘Four o'clock flower’) is crossed with a white Mirabilis jalapa plant, all the offspring flowers are pink because both the red and white alleles are expressed.

What is incomplete dominance?

Sometimes two different alleles are neither fully dominant or recessive to each other.

In heterozygous individuals, this creates a phenotype that is an intermediate mix of the other two. This is called incomplete dominance.

Work through the questions on the sheets.

Outcomes






recessive


BLOOD TYPE IS GENETIC

• The A and B antigen molecules on the surface of red blood cells are produced by two different enzymes. These two enzymes are encoded by different alleles, of the same gene: A and B.

• The A and B alleles code for enzymes that produce the type A and B antigens respectively.

• A third version of this gene, the O allele, codes for a protein that is not functional and does not produce surface molecules.


Construct a diagram to show the possible blood types that

may be inherited.

Calculate the genotype and phenotype

Outcomes






recessive



• Blood plasma is packed with proteins called antibodies. The body produces a wide variety of antibodies that will recognize and attack foreign molecules that may enter from the outside world.

• A person's plasma does not contain any antibodies that will bind to molecules that are part of his or her own body.


WHEN BLOOD TYPES MIX


• When conducting a blood transfusion, it is important to carefully match the donor and recipient blood types.

• If the donor blood cells have surface molecules that are different from those of the recipient, antibodies in the recipient's blood recognize the donor blood as foreign.

• This triggers an immune response resulting in blood clotting.• If the donor blood cells have surface molecules that are the same as

those of the recipient, the recipient's body will not see them as foreign and will not mount an immune response.


SPECIAL BLOOD TYPES

• People with type O blood are universal donors because there are no molecules on the surface of the red blood cells that can trigger an immune response. People with type AB blood are universal recipients because they do not have any antibodies that will recognize type A or B surface molecules.

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1 of 8© Boardworks Ltd 2007 Inheritance terms. Objectives Define key inheritance terms. Determine genotypes and phenotypes of the offspring of a monohybrid