6.3 Mendel and Heredity. 13.1 Ecologists Study Relationships Fundamentals of Genetics Genetics – field of biology devoted to understanding how characteristics

6.3 Mendel and Heredity

13.1 Ecologists Study RelationshipsFundamentals of Genetics

Genetics – field of biology devoted to understanding how characteristics are transmitted from parents to offspring


Father of GeneticsGregor MendelAustrian Monk

• Heredity – transmission of characteristics from parents to offspring

• Garden Peas


Characteristic Traits

1. Plant Height Tall Short

2. Flower position Axial Terminal

3. Pod appearance Inflated Constricted

4. Seed Texture SmoothWrinkled

5. Seed Color Yellow Green

6. Flower Color Purple White

7. Pod Color Green Yellow



Mendel’s Methods

• Pollination – pollen grains produced in the male reproductive parts of the flower (anther) are transferred to the female reproductive part of a flower (stigma)

• Self-pollination – pollen is transferred from the anthers of a flower to the stigma on a flower on the same plant

• Cross-pollination – involves flowers of two separate plants


Mendel’s Experiments• Grew plants PURE for a trait (always produce

offspring with that trait)• STRAIN - plants that are pure for a specific trait• He allowed plants to self-pollinate for several

generations to obtain 14 strains: Parental Generation (P1)

• Cross-pollinated these strains one pure for one trait with another pure for the contrasting trait P1 (tall) X P1 (short) First Filial Generation(F1)

• Then allowed F1 to self pollinate second filial generation (F2)


Mendel controlled thefertilization of his pea plantsby removing the male parts,or stamens.

He then fertilized the femalepart, or pistil, with pollen froma different pea plant.

interrupted the self-pollination process by removing male flower parts


Mendel’s Results

• Only one of the two traits in P1 appeared in the offspring F1

• The trait then reappeared in F2 in a ratio of 3:1


Mendel’s Conclusions

• Factor – something controlling the traits (allele)• Pair of factors controls each trait (gene)• Recessive & Dominant Traits

- Dominant factor – masked the other factor (appeared in F1)

- Recessive – is masked by the presence of another (reappeared in F2)


Chromosomes and Genes• Molecular genetics – study of the structure and

function of chromosomes and genes• Gene – segment of DNA on a chromosome that

controls a particular hereditary trait• Letters are used to represent alleles

- capital letters refer to dominant allelesT = tall

- lowercase letters refer to recessive allelest = short• Genome - All of an organism’s genetic material


Genetic Crosses• Genotype – genetic makeup of an organism • TT Tt tt• Phenotype – appearance of an organism as a

result of its genotypeTT Tall tt short

• Homozygous – both alleles of a pair are alike TT or tt

• Heterozygous – two alleles in the pair are different Tt


• Genotypic ratio – ratio of the genotypes that appear in offspring

1TT:2Tt:1tt

• Phenotypic ratio – ratio of the offspring's phenotypes

3 Tall:1 short


Punnett squares illustrate genetic crosses.

• The Punnett square is a grid system for predicting all possible genotypes resulting from a cross.– The axes represent

the possible gametesof each parent.

– The boxes show thepossible genotypesof the offspring.

• The Punnett square yields the ratio of possible genotypes and phenotypes.

6.5 Traits and Probability


A monohybrid cross involves one trait. (12 points)


Homozygous Dominant X Homozygous Recessive

6.3 Mendel and Heredity6.5 Traits and Probability

Heterozygous X Heterozygous


Testcross

• Individual of unknown genotype is crossed with a homozygous recessive individual

• Determine the genotype of an individual whose phenotype is dominant


Testcross (10 points): Offspring 100% Dom



• Complete Dominance – one allele completely dominant over the other

• Incomplete dominance – two or more alleles influence the phenotype resulting in a phenotype intermediate between the dominant and recessive traitsR=red W=white RW pink


• Codominance – both alleles for a gene are expressed in a heterozygous offspring – neither is dominant or recessive

RW red & white polka dots


Multiple Alleles – 3 or more alleles of the same gene

• Blood Types• IA IB i• IAIA or IAi = Blood type A• IBIB or IBi = Blood type B• IAIB = Blood type AB• ii = Blood type O

• The ABO blood types result from codominant multiple alleles.


Sex-linked Inheritance

• Traits controlled by genes located on the sex chromosomes

• X-linked gene• Y-linked gene• X-linked traits more common in males

Sex-Influenced Inheritance – traits affected by how much sex hormones are present


Punnett Square: Sex-Linked Traits

• Determine Sex of Offspring• Difference in sex-linked traits.

7.1 Chromosomes and Phenotype


• Male mammals have an XY genotype.

– All of a male’s sex-linked genes are expressed.

– Males have no second copies of sex-linked genes.



• Female mammals have an XX genotype.

– X chromosome inactivation randomly “turns off” one X chromosome.



A dihybrid cross involves two traits. (12 points)

Homozygous Dominant for both X

Homozygous Recessive for both



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



Laws• Law of Segregation – a pair of factors is

segregated, or separated, during the formation of gametes (1 trait: Tall from short)

• Law of Independent Assortment – factors for different characteristics are distributed to gametes independently (all characteristics being separated)

i.e. Tall plant from yellow peas


• Mendel’s rules of inheritance apply to autosomal genetic disorders.

– A heterozygote for a recessive disorder is a carrier. – Disorders caused by dominant alleles are uncommon.

(dominant)



Polygenic Inheritance

• Traits controlled by many genes• Multifactorial• ~180 genes have been described to control

human height up to 700• Add the effect of all genes to get the

manifestation of the trait• ABC – each having their own set of alleles• AaBbCc


• An epistatic gene can interfere with other genes.

7.2 Complex Patterns of Inheritance


The environment interacts with genotype.

• Height is an example of a phenotype strongly affected by the environment.

• The sex of sea turtles depends on both genes and the environment

• Phenotype is a combination of genotype and environment.



Many genes may interact to produce one trait.

Order of dominance: brown > green > blue.


Documents

6.3 Mendel and Heredity. 13.1 Ecologists Study Relationships Fundamentals of Genetics Genetics – field of biology devoted to understanding how characteristics