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Describing monohybrid crosses

Science - Genetics

Mr Richardson

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Key principles

The I do phase involves the explanation - indicates where a teacher would be explaining and modelling a concept.

Moving from I do to We do involves guided practice - moving from teacher-led instruction to student-led practice.

Check for Understanding - Formative instruction questions, posed to check whether students have understood and to inform next instructional steps.

You do - Practice - indicates places where students will attempt questions independently.

At the expected level for the lesson; all students should be able to do these questions.

A small increase in difficulty from the expected level, eg larger numbers or a more complex calculation.

Extension - increased difficulty; suitable for a small number of students.

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I

CfU

W

Y

Support - support materials, such as additional templates or questions with fewer steps.

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Engagement icons - to go in top right corner

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Whiteboard response

All students respond to the teacher’s question, showing their responses at the same time.

Non-volunteers

The teacher asks a question, gives wait time, then calls on individual students to respond.

Written response

Students respond to a prompt in a template, their exercise book or booklet.

Pair share

Students discuss with the person next to them.

Choral response

Students respond orally all together.

Thumbs up, thumbs down

Students indicate whether they agree or disagree with a statement, or whether something is an example or non-example.

Concrete materials

Teacher might choose to illustrate this teaching point using concrete materials.

Class discussion or activity

Students discuss as a class and share ideas

Calculator

Students will need a calculator to perform the task on this slide.

Spreadsheet

Students will need access to spreadsheet software to perform the task on this slide.

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In this lesson, you will need:

Exercise book or paper

Pen

Image credit: Pixabay

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Starter Quiz

Genetic crosses

Mendel and Punnett

Gene Dominance

Exit Quiz

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Keywords

Hybrid - a combination of the material from two unique individuals.

Monohybrid cross - the crossing of one gene from two parents.

Punnett square - a square diagram that can be used to predict the genotype of offspring.

Gamete - an organism's reproductive cells or sex cells.

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Keywords

Zygote - fertilized egg cell from a male and female gamete together.

Allele - variant forms of a gene that codes for a particular trait.

Dominant gene - expresses itself more strongly than any other version of the gene which the person is carrying.

Recessive gene - a gene whose effects are masked in the presence of a dominant gene.

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Keywords

Homozygous - two identical alleles of a particular gene.

Heterozygous - two different alleles of a particular gene.

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Fertilisation

Gametes have half the genetic material of the parent (haploid).
A fertilised egg is called a zygote.
A zygote combines both gamete chromosomes to form a diploid cell with genetic material from both parents.

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Allele pairs

Genes are inherited in pairs on chromosome pairs

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All alleles come in pairs

DD

dd

Dd

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Allele inheritance

Parents each pass on one allele to offspring

Each allele from the parent has a 50% chance of being inherited

How do these alleles become inherited in a zygote?

Meet barry and bessie (click) Barry has brown eyes, and bessie has blue eyes. The allele for blue eyes is little b and the allele for brown eyes is big b. In this example, they both have the same allele coded on the gene in their chromosome pairs in their parent cells. These parent cells produce gametes.

We covered meiosis in a previous lesson, so feel free to go back and have a look at the concepts covered there, but for this lesson we are going fast forward to the fact that through meiosis, barry and bessie are going to produce gametes that are haploid and have a 50% chance of carrying either allele in their maternal (mother) and paternal (father) gametes. (click)

When these gametes meet, they fertilise and form a zygote. However that 50% chance from each parent means that each offspring actually has a 25% chance of getting that allele combination

Confused? You’re not alone, we actually didn’t truly understand this until about 100 years ago!

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Pick the true statements

Option 1

Option 2

Option 3

Option 4

Gametes are diploid and have a full set of chromosome pairs.

Gametes are haploid and carry one allele from the parent cell.

Alleles are written as capital and lowercase versions of the same letter.

An allele has a 10% of being passed onto offspring.

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Y

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Mendel and Punnett

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Mendel (1822-1884)

Father of modern Genetics
Experimented on sexual reproduction of pea plants.
Showed the basis of heredity and inheritance using 7 traits.

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Credit: Free SVG

Gregor Mendel (1822–1884) was an Austrian monk, carried out experiments on pea plants in his monastery garden for 17 years, not really into the party life thats for sure. He was a nobody for 35 years and his work wasn’t discovered until after he died. But was unearthed after old Charlie Darwin introduced this idea of evolution and adaptation through inheriting advantageous traits. Then greggy blew up! He took METICULOUS notes and carried out hundreds of experiments by crossing multiple generations of these little pea plants. And scientists jumped on his findings and called him the Father of genetics. (click)

He’s the father of genetics because he was the first person to clearly record patterns of inheritance. He did this by manually fertilising pea plants with specific traits (click) and he focussed in on 7 specific traits. (click)

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The tall and the short of it

Mendel discovered he could breed ‘purebreds’ for a trait.
Crossing these purebreds created hybrids.

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The tall and the short of it

Mendel showed some alleles could be masked by other alleles, these are called recessive alleles.
The monohybrid ratio is the ratio of offspring from two hybrid parents is usually 3:1

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So lets write out what happens when greggy crossed a pure-breeding tall plant with a pure-breeding short plant as parents. (click) We’ve Shown this a P generation and the traits of purebred tall as TT and purebred short tt.

Now it seemed that the Tall allele has overpowered the short allele. And you can see this through F1 generation here (click). Greggy Mendel would get 100% of his plants in this generation Tall. This F in the F1 generation means ‘filial’ which means son or daughter. Fancy geneticist speak again.

This is where it gets interesting, because Mendel then showed the factor for shortness had not disappeared because when he crossed the tall offspring (called the F1 generation) with each other, about a quarter of those offspring (called the F2 generation) were short. (click) He called shortness a “recessive factor” because it was hidden or masked in the F1 generation. A plant is a hybrid if it has parents with both alternatives, such as tallness and shortness, for a characteristic. And THIS is where that term mongrel comes in.

(click)

We now know that Mendel’s ‘factors’ are genes. The alternative forms of the factors are alleles. He worked out that if many pure-breeding tall and short plants were crossed and then the first generation (F1 generation) was also crossed, the ratio of tall to short plants would be about 3 :1. He repeated these experiments many times using the other characteristics of the pea plants and came up with similar ratios (his exact ratios were 787 Tall: 277 short , a ratio of 2.84:1) which is pretty amazing considering DNA had only been identified as a thing a few years earlier and Mendel hadn’t even heard about it in the monastery. This ratio is called the monohybrid ratio. Mono meaning one, hybrid meaning mix.

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Punnett (1875-1967)

Eccentric uncle of modern Genetics
Created the Punnett Square to model monohybrid ratio

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Ratio 3:1
Percentage 75% to 25%
Or ¾ to ¼

If o’l Gregor is the father of genetics, Reginald Punnett is the eccentric uncle. (click) Reggie was from Britain, and went to the very well known university of Cambridge. He loved ball sports and breeding things and bred worms, chickens, maize (a type of food crop) and pea plants for his academic career as a geneticist. Reggie loved Greggy and applied some maths to mendelian genetics which was just becoming ‘discovered’ in the academic world when Reg was studying at uni. Reg decided to take something cool and new and make it even funner, and added some maths to it. This is pretty easy maths though so don’t panic. In fact we are just going to call it a model. This model shows the monhybrid ratio and further confirms what we know about the 50% chance of offspring getting alleles from parents in their gametes.

You’ll also see that the Tall allele overpowers or masks the short allele so it still confirms that idea that Mendel had about recessive traits.

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Mendel summary: mark where the following occur

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Y

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Mendel summary: mark where the following occur

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Punnett summary: fill in the blanks

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Parent Gametes	?	b
B	BB	?
b	Bb	?

Y

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Punnett summary: fill in the blanks

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Parent Gametes	B	b
B	BB	Bb
b	Bb	bb

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Which of the following doesn’t describe a monohybrid ratio?

Option 1

Option 2

Option 3

Option 4

3:1

60% to 40%

¾ to ¼

75% to 25%

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Y

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Gene dominance

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Recessive and dominant traits

Recessive allele:

Masked by the presence of a dominant allele
Not expressed physically unless both alleles are recessive
Lowercase letter

Dominant allele:

Always expressed if present
Capital letter

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Credit: Free SVG

Why do some people have traits that neither of their parents don’t? Like this little red headed baby? Neither of his parents are red headed. He’s got blue eyes like his mum and sister…but that doesn’t explain hsi red hair.

Remember mendel? And the hybrid parents? Hybrid means mongrel remember, and this means they have two alleles for a trait.

Usually these alleles are described as recessive (click) which means The recessive trait is hidden in the presence of the dominant trait and can be expressed only if the allele for the dominant trait is not present and both are recessive (click). This is the reason we write them as lowercase letters (click) Mystery solved!

What about dominant alleles? (click) pretty much If the allele for a dominant trait is present, it will always be expressed. (click) and this is why it goes first and is a capital letter to show it’s dominance. (click)

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Homozygous and heterozygous

Two alleles the same is called homozygous
Two different alleles (recessive and dominant) is called heterozygous

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Heterozygous

Homozygous

Dominant trait expressed

Recessive trait expressed

DD

bb

Aa

AA

Zz

Rr

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Sort: Gene dominance

Y

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Heterozygous

Homozygous

Dominant trait expressed

Recessive trait expressed

DD

bb

AA

Zz

Rr

Aa

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Sort: Heterozygous - Homozygous

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