+

Genetics Recap

+ObjectivesTo be able to: Explain how characteristics are inherited from parents

and how genetic variations occur. Demonstrate the characteristics that will be inherited

by offspring using punnett squares. Differentiate between a monohybrid cross & a dihybrid

cross.

+The cell’s nucleus contains chromosomes. These are long threads of DNA, each made up of many genes

A gene is a section of DNA that codes for a specific protein.

+

Some characteristics, such as eye colour and the shape of the earlobe, are controlled by a single gene. These genes may have different forms.

Different forms of the same gene are called alleles. The gene for eye colour has an allele for blue eye colour and an allele for brown eye colour.

+Alleles

Alleles are dominant or recessive:

• the characteristic controlled by a dominant allele develops if the allele is present on one or both chromosomes in a pair

• the characteristic controlled by a recessive allele develops only if the allele is present on both chromosomes in a pair

+A monohybrid cross shows the inheritance of one characteristic. In the genetic diagrams for these crosses:

• the recessive allele is represented by a lower case letter

• the dominant allele is represented by an upper case letter

• someone with two identical copies of an allele is homozygous for that particular gene

• someone with two different alleles for a particular gene is heterozygous for that gene

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+Mendel’s experiment

Gregor Mendel (1822-1884) studied the inheritance of different characteristics in pea plants. He found that when he bred purple-flowered plants with white-flowered plants, all the offspring produced purple flowers.

If he bred these plants with each other, most of the offspring had purple flowers, but some had white. This was because the allele for purple flowers is dominant, and the allele for white flowers is recessive.

+Punnett SquaresA punnett square shows how chromosomes or alleles may combine in zygotes.

+Mendel’s first cross

+

homozygous dominant X

homozygous recessive =

all heterozygous, all dominant

(100% heterozygous dominant)

Mendel’s first cross

+Mendel’s second cross

+Mendel’s second cross

heterozygous X heterozygous = 1:2:1

homozygous dominant: heterozygous: homozygous

recessive

(3:1 = dominant : recessive)

+

A dihybrid cross shows the inheritance of two characteristics.

In a dihybrid cross the genes that determine the two characteristics are located on different pairs of homologous chromosomes. Each of these genes can have two or more alleles.

+YR Yr yR yr

YR

Yr

yR

yr

Y = yellowy = greenR = roundr = wrinkled

+

+A dihybrid cross involves two traits. Mendel’s dihybrid crosses with heterozygous plants

yielded a 9:3:3:1 phenotypic ratio.

• Mendel’s dihybrid crosses led to his second law,the law of independent assortment.

• The law of independent assortment states that allele pairs separate independently of each other during meiosis.

+Heredity patterns can be calculated with probability.

Probability is the likelihood that something will happen. Probability predicts an average number of occurrences, not an exact

number of occurrences.

• Probability =number of ways a specific event can occur

number of total possible outcomes

• Probability applies to random events such as meiosis and fertilization.

+Sexual reproduction creates unique combinations of genes. Sexual reproduction creates unique combination of genes.

independent assortment of chromosomes in meiosis random fertilization of gametes

Unique phenotypes may give a reproductive advantage to some organisms.

+Crossing over during meiosis increases genetic diversity. Crossing over is the exchange of chromosome segments

between homologous chromosomes. occurs during prophase I of meiosis I results in new combinations of genes

• Chromosomes contain many genes.

– The farther apart two genes are located on a chromosome, the more likely they are to be separated by crossing over.

– Genes located close together on a chromosome tend to be inherited together, which is called genetic linkage.

• Genetic linkage allows the distance between two genes to be calculated.