unit 2: genetic processes chapter 4: cell division and reproduction chapter 5: patterns of...

UNIT 2: Genetic Processes

Chapter 4: Cell Division and Reproduction

Chapter 5: Patterns of Inheritance

How are traits inherited, and how can inheritance be predicted?

Chapter 6: Complex Patterns of Inheritance

5: Patterns of Inheritance

Canola (Brassica napus) is a Canadian success story. It was developed in a traditional selective breeding program in the 1970s. It is now a valuable Canadian crop that benefits from continued modern, molecular genetics research.

Chapter 5: Patterns of InheritanceUNIT 2

5.1 Understanding Inheritance While people bred animals and plants for thousands of years without understanding the mechanisms of inheritance, eventually theories and explanations of how breeding worked were proposed.

The first widely accepted theory was pangenesis, proposed by Aristotle. It suggested that sperm and egg contained tiny particles from all body parts.

Others thought that only the sperm had such an essence. In fact, it was proposed that an entire miniature human being was inside the sperm!

By the 1800s, people settled on the idea that traits from the parents were irreversibly blended in the offspring.

None of these theories was based on scientific evidence.

UNIT 2 Chapter 5: Patterns of Inheritance Section 5.1

Gregor Mendel’s ExperimentsGregor Mendel (1822–84), an Augustinian monk, used scientific methods to solve the mystery of how traits were inherited. Before his time at the monastery, he studied botany and mathematics, which proved invaluable to his observations.

One of the keys to his discovery was the plant type he chose to work with: pea plants. Pea plants come in many varieties and show different traits (characteristics exhibited by an organism). In addition, they usually self-fertilize, which allowed Mendel to start with plants that were true breeding (same outcome traits every generation). He carefully cross-pollinated true-breeding pea plants.


Between 1856 and 1863, Mendel bred, tended, and analyzed more than 28 000 pea plants in his monastery garden.

Mendel’s Monohybrid ExperimentsMendel started every experiment with plants that were true breeding for a trait but that exhibited a different form of the trait. He called this the parental, or P generation. Offspring were called the first filial (F1) generation. These experiments were called monohybrid crosses because only one (mono) trait was monitored at a time. However, Mendel studied seven different traits in his experiments.


Mendel’s Results Example 1: P generation of male yellow-pea-producing plant and female green-pea-producing plant

P generation cross results: All offspring (F1 generation) were the same seed colour: yellow, i.e., one parent’s seed colour trait seemed to disappear. This result was the same for each of the seven traits he studied.


Continued…

Mendel’s Results Example 2: F1 generation of yellow-pea producing plants

F1 generation cross results: In the F2 generation, some peas were yellow and some green. Mathematically, the ratio was 3:1 yellow:green. This ratio was the same for all seven traits that Mendel studied.


The Law of SegregationMendel concluded that there must be two hereditary “factors” for each trait. Today we call those factors “alleles.” Recall that diploid organisms have two alleles for each gene.

He also concluded that one factor/allele is always dominant, and one is recessive. In the example, yellow colour is dominant over green when it comes to the colour of seeds in the pea plant.

Mendel proposed the “law of segregation” to explain this: Traits are determined by pairs of alleles that segregate during meiosis so that each gamete receives one allele (updated terminology).


Genotype and PhenotypeTo express alleles easily in written form, upper and lower case letters are used. A dominant allele is represented by the first letter of the allele’s description. The recessive allele then receives the lower case of the same letter.

Yellow pea allele: Y Green pea allele: y

In each plant, two alleles are present so the possible combinations are:

YY, Yy, or yy. This is the plant’s genotype.

The actual colour of the peas is the plant’s phenotype.


Section 5.1 Review


5.2 Studying Genetic CrossesThe possibility of a certain allele packaged in a gamete is ½ since there are two alleles in a diploid cell and only one is packaged in a haploid gamete.


Thus, when determining the possible outcomes of a monohybrid cross, there is ½ X ½ = ¼, or a 25% chance of each combination of alleles in the offspring. We use a grid called a Punnett square to show the law of segregation and possible cross outcomes.

Using Punnett SquaresA Punnett square demonstrates the possible F1 outcomes from a cross between two heterozygous parents. In this case, the parents are heterozygous for flower colour. The phenotype ratio is 3:1 for flower colour (purple to white).


Test CrossesWhen geneticists want to know if an individual is heterozygous or homozygous for a dominant phenotype, they do a test cross. A test cross is a cross between an individual of unknown genotype for a trait and an individual that is homozygous recessive for that trait. Analyzing the phenotype should provide insight into the unknown genotype.


In a test cross, if any of the offspring show the recessive phenotype, the unknown genotype of the parent must be heterozygous.

What do you think the F2 generation looked like? Explain your answer.

Mendel’s Dihybrid CrossesMendel also designed experiments to follow the inheritance pattern of two traits to determine if the inheritance of one trait affected another.

He crossed true-breeding plants that produced yellow, round seeds (YYRR) with true-breeding plants that produced green, wrinkled seeds (yyrr). The peas in the F1 generation all displayed the dominant trait for both traits (yellow and round).


Continued…

Mendel’s Dihybrid Crosses The F1 generation self-fertilized to create the F2 generation. It had a mix of four phenotypes but came close to the ratio 9:3:3:1 (yellow, round to yellow, wrinkled to green, round to green, wrinkled).


Mendel’s Results A Punnett square can show the segregation of the gametes for two traits. Each parent can package the alleles in the gametes in four different ways.


The Law of Independent AssortmentMendel found the 9:3:3:1 ratio for every dihybrid cross he performed. This is expected only if the inheritance of one trait has no influence on the inheritance of another trait. He described these events in the law of independent assortment. Using current terminology, this law states that the alleles for one gene segregate or assort independently of the alleles for other genes during gamete formation.


The Chromosome Theory of Inheritance When Mendel performed his experiments and formulated his laws of inheritance, the process of meiosis and the existence of chromosomes had not been discovered. By the early 1900s, scientists began to see the link between both.


Continued…

The Chromosome Theory of Inheritance In 1902, Walter Sutton showed that the behaviour of chromosomes during meiosis was related to the behaviour of Mendel’s factors. He realized that during gamete formation, alleles segregate just as homologous chromosomes do, and proposed that genes are carried on chromosomes. This formed the basis of the chromosome theory of inheritance: Genes are located on chromosomes, and chromosomes provide the basis for the segregation and independent assortment of alleles.


Section 5.2 Review


5.3 Following Patterns of Inheritance in Humans

Geneticists who study human inheritance collect as much information as they can and use it to create a diagram called a pedigree. A pedigree is a type of flow chart that uses symbols to show the inheritance patterns of traits in a family over many generations. They help uncover the genotype of a particular member of a family, and they can be used to predict phenotypes and genotypes of future offspring.


How is human genetic research different from genetic research on plants and animals?

Continued…

Following Patterns of Inheritance in Humans


Autosomal InheritanceAutosomal inheritance refers to the inheritance of traits whose genes are found on the autosomes (chromosomes 1 – 22). These chromosomes hold normal, functioning genes (hair colour, freckles) as well as disorder genes (cystic fibrosis, Huntington disease).

An autosomal dominant disorder occurs when the disease-causing allele is dominant and an individual has one or both copies of the allele. An autosomal recessive disorder occurs when the disease-causing allele is recessive and an individual has both copies of the allele.


Autosomal InheritanceWhen using a pedigree to study a disorder, you can determine if the pattern is autosomal dominant or autosomal recessive.


Huntington Disease: Autosomal Dominant

An unaffected child born of two affected parents indicates autosomal dominantinheritance.

This pedigree shows the inheritance pattern for an autosomal dominant disorder. Notice that an affected child must have at least one affected parent to be affected.

Autosomal InheritanceUNIT 2 Chapter 5: Patterns of Inheritance Section 5.3

Cystic Fibrosis: Autosomal Recessive

In autosomal recessive inheritance, if both parents are heterozygous for thedisorder, they will have an affected child. This pedigree shows the inheritance pattern

for an autosomal recessive disorder.Notice that the appearance of the recessive phenotype can skip generations, and that twounaffected parents can have an affected child.

Tests for Genetic Diseases


Genetic Counselling


A genetic counsellor has special training in human genetics and in counselling. A family may seek a counsellor when there is a history of a genetic disorder in the family. Counsellors often use pedigrees to determine offspring risk.

Section 5.3 Review


unit 2: genetic processes chapter 4: cell division and reproduction chapter 5: patterns of...

Documents

patterns of inheritancesection

f1 generation of yellow

offspring f1 generation

f2 generation

patterns of inheritanceunit

filial f1 generation

outcome traits

true breeding