Mendelian Genetics, Meiosis and Evolution

• Mendel’s Laws of Heredity (10.1)• Meiosis (10.2)

• The Theory of Evolution (15)

Answer these questions… What is heredity?

The passing of traits from parents to offspring What is genetics?

The branch of biology that studies heredity What is sexual reproduction?

Male sex cell + Female sex cell Fertilized cell Male gamete + Female gamete Zygote

What is the above process called? Fertilization

In a plant, what does a zygote develop into? A seed

What is pollination? The transfer of pollen grains from a male reproductive organ

to a female reproductive organ in a plant What is a hybrid?

The offspring of parents that have different forms of a trait

Gregor Mendel

Old thinking: Blending Traits from both parents

get blended and the offspring is a mixture of both

Cross-pollinated pea plants to study how traits are passed from parents to offspring

Mendel’s 3 Conclusions

The rule of unit factors Each organism has two factors that control each of

its traits Factors = genes (located on chromosomes) Genes exist in alternative forms

Alleles Example: A pea plant could have 2 alleles for purple,

2 alleles for white or 1 allele for purple and 1 allele for white

An organism’s two alleles are located on different copies of a chromosome (one from the female parent, one from the male)

Mendel’s 3 Conclusions The rule of dominance

Purple flower + White flower = Purple flower Only the purple trait is observed, therefore it is

dominant The white trait ‘disappeared’, therefore it is

recessive

The law of segregation Every individual has two alleles of each gene When gametes (sex cells) are produced each

gamete receives one of these alleles During fertilization, these gametes randomly pair

to produce four combinations of alleles

Answer these questions…

What are the alleles of the purple-flowered plant in the parental generation? 2 for purple

The white-flowered? 2 for white

The first gen. offspring? 1 for purple, 1 for

white

Phenotypes and Genotypes The purple parent has 2 purple alleles and

the purple offspring has 1 purple and 1 white allele

Therefore, two organisms can look alike, but have different underling allele combinations

The way an organism looks and behaves is called its phenotype

The allele combination an organism contains is called its genotype

Phenotypes and Genotypes

If an organisms alleles are the same for a trait (think purple parent) then the organism is homozygous for that trait

If an organisms alleles are different for a trait (think first gen. purple offspring) then the organism is heterozygous for that trait

Mendel’s conclusions allow us to… Predict the probability of the genotype of an

offspring when given the parents genotype Determine a phenotype based on genotype

Answer this question…

If two plants are crossed that have two different traits, will the two traits stay together or will they be inherited independently of each other?

The Law of Independent Assortment

Genes for different traits are inherited independently of each other

Answer these questions… Where are genes located?

Chromosomes How many chromosomes do humans have?

46 (23 pairs) How many chromosomes in each cell?

46 (23 pairs) Where do these chromosomes come from?

23 from mom, 23 from dad Which cells combine to form a zygote (and then

offspring)? Gametes (sex cells)

Therefore, how many chromosomes in a gamete? 23

Diploid and Haploid Cells A cell with pairs of each chromosome is called

a diploid cell (2n) A cell with one of each chromosome is called a

haploid cell (n)

The two chromosomes of each pair in a diploid cell are called homologous chromosomes Not identical Contain information for the same traits Can have different alleles

Typical Animal Life Cycle

Homologous Chromosomes (Homologs)

Chromosomes 1 & 2 are homologous chromosomes

Chromosomes 3 & 4 are homologous chromosomes

Chromosomes 1 & 3 came from the mother

Chromosomes 2 & 4 came from the father

Meiosis – Overview

Meiosis is a special type of cell division that occurs in sexually reproducing organisms Chromosome number reduced by half, enabling

sexual recombination to occur. Meiosis of diploid cells haploid daughter cells (which

may function as gametes) Gametes undergo fertilization, restoring the diploid

number of chromosomes in the zygote

Meiosis – Overview

Meiosis and fertilization introduce genetic variation in three ways: Crossing over between homologous

chromosomes at prophase I Independent assortment of homologous pairs

at metaphase I Each homologous pair can orient in either of two ways

at the plane of cell division Random chance fertilization between any one

female gamete with any other male gamete

Meiosis - Overview

Sexual reproduction in a population should decline in frequency relative to asexual reproduction Asexual – No males are needed, all individuals

can produce offspring Sexual – Only females can produce offspring,

therefore fewer are produced Sexual reproduction may exist because it

provides genetic variability that reduces susceptibility of a population to pathogen attack This is the role of sexual reproduction in

evolution

Meiosis

2 main stages Meiosis I

Prophase I, Metaphase I, Anaphase I, Telophase I

Meiosis II Prophase II, Metaphase

II, Anaphase II, Telophase II

Meiosis I – Prophase I The chromosomes condense and

become visible The centrioles form and move toward

the poles The nuclear membrane begins to

dissolve The homologs pair up, forming a

tetrad Each tetrad is comprised of four

chromotids - the two homologs, each with their sister chromatid

Homologous chromosomes will swap genetic material in a process known as crossing over Crossing over serves to increase genetic

diversity by creating four unique chromatids

Meiosis I – Metaphase I

Microtubules grow from the centrioles and attach to the centromeres

The tetrads line up

along the cell equator

Meiosis I – Anaphase I

Homologous chromosomes separate (note that the sister chromatids are still attached)

Cytokinesis begins

Meiosis I – Telophase I

The chromosomes may decondense (depends on species)

Cytokinesis reaches

completion, creating two haploid daughter cells

Meiosis II – Prophase II

Centrioles form and move toward the poles

The nuclear

membrane dissolves

Meiosis II – Metaphase II

Microtubules grow from the centrioles and attach to the centromeres

The sister chromatids

line up along the cell equator

Meiosis II – Anaphase II

The centromeres break and sister chromatids separate 

Cytokinesis begins

Meiosis II – Telophase II

The chromosomes may decondense (depends on species)

Cytokinesis reaches

completion, creating four haploid daughter cells

https://www.youtube.com/watch?v=D1_-mQS_FZ0

Answer these questions… How does the number of daughter cells produced

from mitosis and meiosis differ? When mitosis is complete, there are two daughter cells. When

meiosis is complete, there are four. How does the ploidy of the daughter cells produced

from mitosis and meiosis differ? Mitosis produces diploid (2n) cells. Meiosis produces haploid (n)

cells. Do the daughter cells produced from mitosis contain

identical genetic complements? Yes, the purpose of mitosis is to produce two identical cells

Do any of the daughter cells produced from meiosis contain identical genetic complements? No, the genetic information swapped between homologous

chromosomes during crossing over insures that each daughter cell produced during meiosis will be unique

Answer these questions… When do the homologous chromosomes

separate during mitosis? Never, they are never joined during mitosis (no tetrads are

formed) When do the homologous chromosomes

separate during meiosis? Homologs separate during Anaphase I, when the tetrads

break When do sister chromatids separate during

mitosis? Sister chromatids separate during Anaphase.

When do sister chromatids separate during meiosis? Sister chromatids separate during Anaphase II.

The Consequences of Meiotic Mistakes

Nondisjunctions occur when homologous chromosomes fail to separate at meiosis I or when chromatids fail to separate at meiosis II. 

The Consequences of Meiotic Mistakes Nondisjunctions occur when homologous

chromosomes fail to separate at meiosis I or when chromatids fail to separate at meiosis II Fertilization can result in embryos that are 2n + 1 (a

"trisomy") Abnormal copy numbers of one or more chromosomes is

usually, but not always, fatal (Example: Down syndrome)

Polyploidy can occur when whole sets of chromosomes fail to separate at meiosis I or II The resulting 2n gametes, if fertilized by normal sperm,

create 3n zygotes (triploid) Organisms with an odd number of chromosome sets

cannot produce viable gametes (Example: seedless fruits)

Answer these questions…

How do mutations drive evolution? Mutations change traits of and organism if

change helps the organism survive the greater the chance of that organism living long enough to reproduce trait gets passed down through generations

How does sexual reproduction drive evolution? Sexual reproduction may exist because it provides

genetic variability that reduces susceptibility of a population to pathogen attack

Evolution All of the similarities

and dissimilarities among groups of organisms that are the result of the branching process creating the great tree of life, were viewed by early 19th century philosophers and scientists as a consequence of omnipotent design.

Evolution

In 1859, Charles Darwin published his famous On the Origin of Species

Patterns in the distribution and similarity of organisms had an important influence of Darwin's thinking

Darwin’s Theory of Evolution

Species (populations of interbreeding organisms) change over time and space.  The representatives of species living today differ from those that lived in the recent past, and populations in different geographic regions today differ slightly in form or behavior.  These differences extend into the fossil record, which provides ample support for this claim.

Darwin’s Theory of Evolution All organisms share common ancestors with

other organisms.  Over time, populations may divide into different species, which share a common ancestral population.  Far enough back in time, any pair of organisms shares a common ancestor.  For example, humans shared a common ancestor with chimpanzees about eight million years ago, with whales about 60 million years ago, and with kangaroos over 100 million years ago.   Shared ancestry explains the similarities of organisms that are classified together: their similarities reflect the inheritance of traits from a common ancestor. 

Darwin’s Theory of Evolution

Evolutionary change is gradual and slow in Darwin’s view.  This claim was supported by the long episodes of gradual change in organisms in the fossil record and the fact that no naturalist had observed the sudden appearance of a new species in Darwin’s time. 

Since then, biologists and paleontologists have

documented a broad spectrum of slow to rapid rates of evolutionary change within lineages. 

The primary mechanism of change over time is natural selection

The Process of Natural Selection Variation

Organisms (within populations) exhibit individual variation in appearance and behavior Body size, hair color, facial markings, voice properties

etc. Some traits show little to no variation among

individuals Number of eyes in vertebrates

Inheritance Some traits are consistently passed on from

parent to offspring Other traits are strongly influenced by

environmental conditions and show weak heritability

The Process of Natural Selection

High rate of population growth Most populations have more offspring each year

than local resources can support leading to a struggle for resources. 

Each generation experiences substantial mortality.

Differential survival and reproduction Individuals possessing traits well suited for the

struggle for local resources will contribute more offspring to the next generation. 

Final Thoughts on Evolution

In order for natural selection to operate on a trait, the trait must possess heritable variation and must confer an advantage in the competition for resources.  If one of these requirements does not occur, then the trait does not experience natural selection

“…as natural selection acts by competition for resources, it adapts the inhabitants of each country only in relation to the degree of perfection of their associates” (Charles Darwin, On the Origin of Species, 1859).

Final Thoughts on Evolution

Variations arise by mutation  Mutations arise by chance and without

foresight for the potential advantage or disadvantage of the mutation. 

In other words, variations do not arise

because they are needed.