Science Explorer CH. 3

Generics: The Science of Heredity

Mendel’s Work

• Gregor Mendel- was a young priest, taught at local high school and cared for monastery’s garden.

– Wondered why the pea plants had different physical characteristics.

• Traits- a characteristic that an organism can pass on to its offspring through its genes.

• Heredity- the passing of traits from parents to offspring.

– His work formed the foundation of genetics, the scientific study of heredity.

Mendel’s Peas

• Mendel spent over ten years experimenting with the Pea plants.

• Made a wise decision picking the Pea plants.

• Many of the traits only exist in two forms.

– For examples-

• plant stems are either short or tall.

• Produce large amounts of offspring in one generation.

Mendel’s Experiment

• Mendel started the experiment by using purebred plants.– Purebred- is a plant that always produce offspring

with the same form of a trait as the parent.• Short produces short and tall produces tall.

• Used opposite forms of purebred plants.– First experiment Mendel, bred a purebred short with

a purebred tall pea plant.• He called the parent plants parental generation or P

generation.• The offspring from this cross was called first filial generation,

or F1 generation.

Other Traits

• In addition to stem height, Mendel studied six other traits in garden peas:– Seed shape, Seed color, Seed coat color, Pod

shape, Pod color, and Flower position.

• Just like in the stem experiment, only one form of the trait appeared in the F1generation. However, in the F2 generation the “lost” form of the trait always reappeared in about one forth of the plants.

Dominant and Recessive Alleles

• Mendel reasoned that individual factors must control the inheritance of traits in peas.– The factors that control each trait exist in pairs.

– The female parent contributes one factor, while the male parent contributes the other factor.

• Mendel reasoned that one factor in a pair can mask, or hide, the other factor. – The tallness factor, for example, masked the

shortness factor in the F1 generation.

• Genes- a segment of DNA on a chromosome that codes foe a specific trait.

• Alleles- the different forms of a gene.– Each pea plant inherits a combination of two

alleles from its parents- either two alleles for tall stems, two alleles for short stems, or one of each.

• Individual alleles control the inheritance of traits. Some alleles are dominant, while other alleles are recessive.

• Dominant allele- is one whose trait always shows up in the organism when the allele is present.

• Recessive allele- is masked, or covered up, whenever the dominant allele is present.– A trait controlled by a recessive allele will only

show up if the organism does not have a dominant allele.

– Only pea plants that inherits two recessive alleles for short stems will be short.

Understanding Mendel’s Crosses

• When you mate a long stem and a short stem pea plant together, you will get a long stem offspring with the F1 generation. With the F2

generation you will get ¾ tall stem and ¼ will be short stem pea plant.

• Hybrids- an organism that has two different alleles for a trait; an organism that is heterozygous for a particular trait.

Symbols in Genetics

• Geneticists today use standard short hand method.

– The dominant allele will be represented by a capital letter.

– The recessive allele is represented by a lower case letter.

– When both alleles are represented such as a hybrid there will be one capital letter and one lower case letter.

• Mendel presented his findings in 1866. Most of the scientist that read the paper said he over simplified the process of heredity. Others didn’t even bother reading his findings.

• It wasn’t until 34 years later that his findings was considered valid. Many of Mendel’s principles discovered still stand today. That is why he is called the father of Genetics.

• Homework p.85 All

Probability and Genetics

• Principles of Probability

– Probability- is the likelihood that a particular event will occur.

– The laws of probability predict what is likely to occur, not necessarily what will occur.

– However, the more tosses you make, the closer your actual results will be to the results predicted by probability.

– When you toss a coin more than once, the results of one toss do not affect the results of the next toss.

Mendel and Probability

• Mendel realized that he could use probability to predict the type of pea plants that will be produced when he crossed certain types of plants.– When Mendel crossed two hybrid pea plants

together, he found that ¾ of the F1 generation would be tall stem and ¼ of F1 will be short stem.

• Mendel was the first scientist to recognize that the principles of probability can be used to predict the results of genetic crosses.

Punnett Squares

• Punnett Squares-is a chart that shows all the possible combinations of alleles that can result from a genetic cross.

• Geneticists use Punnett squares to show all the possible outcomes of a genetic cross and to determine the probability of a particular outcome.

Using a Punnett Square

• Can use the Punnett to calculate the probability that offspring with a certain combination of alleles will result.

• When you cross two hybrids together the punnett square will result in a 3:1 ratio.

Phenotypes and Genotypes

• Phenotype- is its physical appearance, or its visible traits.– Pea plants can have one of two different

phenotypes for stem height- short or tall.

• Genotypes- is its genetic makeup, or allele combinations.– Homozygous- an organism that has two identical

alleles for a trait.

– Heterozygous- an organism that has two different alleles for a trait.

Codominance

• For all of the traits that Mendel studied, one allele was dominant while the other was recessive. This is not always the case. For some alleles, an inheritance pattern called codominance exists.

• Codominance- the alleles are neither dominant nor recessive.– All the codominant alleles are written as capital

letters with superscripts- FB.

Chromosomes and Inheritance

• Chromosome Theory of Inheritance- genes are carried from parents to their offspring on chromosomes.

• Genes are located on chromosomes.

• Grasshoppers have 24 chromosomes.

– The fertilized egg gets 12 chromosomes from the female and 12 from the male parent.

Meiosis

• Meiosis- is the process by which the number of chromosomes is reduced by half to form sex cells-sperm and egg.

• During meiosis, the chromosome pairs separate and are distributed to two different cells. The resulting sex cells have only half as many chromosomes as the other cells in the organism.

• When sex cells combine to produce offspring, each sex cell will contribute half the normal number of chromosomes. Thus, the offspring gets the normal number of chromosomes- half from each parent.

Chromosomes

• Different animals have different numbers of chromosomes.

– Humans- have 23 pairs or 46 chromosomes.

– Dogs- have 39 pairs or 78 chromosomes.

– Silkworms- have 28 pairs or 56 chromosomes.

Genetic Code

• Today scientists know that the main function of genes is to control the production of proteins in the organism’s cells. Proteins help determine the size, shape, and many other traits of an organism.

• Amino Acids- are the building blocks of proteins.

How Cells Make Proteins

• Protein Synthesis- is the production of proteins.

• During protein syntheses, the cell uses information from a gene on a chromosome to produce a specific protein.

– Takes place on the ribosomes in the cytoplasm of the cell.

The Role of RNA

• Before protein synthesis can take place, a “messenger” must first carry the genetic code from the DNA inside the nucleus into the cytoplasm. This genetic messenger is called ribonucleic acid or RNA.

• RNA molecule almost always looks like only one side, or strand, of the ladder.

• Messenger RNA- copies the coded message from the DNA in the nucleus, and carries the message into the cytoplasm.

• Transfer RNA- carries amino acids and adds them to the growing protein.

Translating the Code

• The first step is for a DNA molecule to “unzip” between its base pairs. Then one of the strands of DNA directs the production of a strand of messenger RNA. To form the RNA strand, RNA bases pair up with the DNA bases. Instead of thymine, however, uracil pairs with adenine. The messenger RNA then leaves the nucleus and attaches to a ribosome in the cytoplasm.

Mutations

• Mutations- is any change in a gene or chromosome.

• Organism’s traits, or phenotype, will be different from what it normally would have been.

Types of Mutations

• Some mutations are the result of small changes in an organism’s hereditary material, such as the substitution of a single base for another.– This can change hair color.

• Other mutations may occur when chromosomes don’t separate correctly during meiosis.– Genetic disorders

• If a mutation occurs in a body cell, such as a skin cell, the mutation will affect only the cell that carries it. If, however, a mutation occurs in a sex cell, the mutation can be passed on to an offspring and affect the offspring’s phenotype.

The Effects of Mutations

• Some of the changes brought about by mutations are harmful to an organism. Other mutations, however, are helpful, and still others are neither harmful or helpful.

– Mutation is harmful to an organism if it reduces the organism’s chance for survival and reproduction.

– Whether a mutation is harmful or not depends partly on the organism’s environment.

• Homework

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