Inherited or Acquired?

Take two cuttings from the same bigleaf hydrangea bush and plant them in two gardens. When they bloom, one can have blue flowers and the other pink.

The two cuttings have the same genes. So where did the difference come from? And a second puzzle: Doctors think one kind of small extra finger is passed on by a dominant allele, yet it is rare. How can a dominant trait be rare?

Explore: grow a garden of genes and soil

In the simulation you grow plants like hydrangeas. You choose a plant’s alleles: a color allele, or a white allele that blocks color. You slide the soil from acidic to less acidic. Then you cut a parent’s stem or dye its petals and grow its seeds.

1. The parent plant’s pair of alleles

Two color alleles Two white alleles

2. Soil pH: 5.0 (lower = more acidic) acidicless acidic

3. Change the parent

Cut the stem Dye the petals

Grow seeds Reset

Parent’s flowers

blue

Young plants’ flowers

not grown yet

In this sketch, passed on from the parent’s change

not grown yet

Pick an allele and slide the soil. Then change the parent and press Grow seeds.

Made up for this sketch: the soil moves in steps of 0.1 pH from 5.0 to 6.5. Blue is shown at 5.5 or lower, in between from 5.6 to 5.9, and pink from 6.0 up; the lesson’s table says “about”, so the edges here are our choice. Six young plants are drawn. Both parents have the same pair, so each young plant gets one allele from each. Nothing here is measured, and the picture is a sketch of the logic, not to scale.

The genes set which colors are possible, and the soil helps decide which one appears. A cut or a dye changes the parent’s body, not the genes in its sex cells, so the young do not get it.

Try this.

1. Choose a plant with two color alleles. Predict what its flowers will do as you slide the soil from acidic to less acidic. Then test it.
2. Switch to two white alleles. Predict whether the soil slider still changes the color. Then test it.
3. Dye a parent’s petals, or cut its stem, and grow its seeds. Predict what the young plants will look like. Then check several of them.

Explain: what can be passed on

Scientists describe living things by features like color, shape, body covering and height. Look closely at any group of the same kind, and each one is a little different. Variation is the small differences among living things of the same kind.

An inherited trait is a trait passed from parents to offspring through genes. Eye color and the shape of a pea seed are inherited traits. An acquired trait is a characteristic a living thing gets during its life. A scar, dyed hair, a skill like playing piano, and muscles built by exercise are acquired traits. The language you speak is acquired too.

Why? A new animal grows from sex cells, an egg and a sperm, which carry the parent’s genes. A scar changes skin cells, and lifting weights changes muscle cells. Neither changes the genes in your sex cells, so neither reaches your children. Changes in the DNA of body cells are not passed on either, but changes in sex cells can be, as that lesson explains.

An old idea, often linked to the naturalist Jean-Baptiste Lamarck, said giraffes stretched their necks and passed longer necks to their young. Experiments do not support it.

Explain: dominant does not mean common

You get one allele, or version of a gene, from each parent (review it in that lesson). Your genotype is the pair of alleles you have. Your phenotype is the trait that shows, such as purple or white flowers.

A dominant allele shows its trait even when you have only one copy. A recessive allele shows its trait only when both of your copies are recessive. When a dominant allele is there, it masks the recessive one. These rules, first worked out with peas by Gregor Mendel, explain how many inherited traits are passed on. Later courses add more patterns.

Dominant describes what happens inside one living thing. It does not tell you how many living things carry that allele.

Word	What it tells you	What it does not tell you
Dominant	One copy is enough for the trait to show	How many people have the trait
Recessive	Two copies are needed for the trait to show	Whether the trait is rare

Explain: genes and environment work together

The environment is everything around a living thing that can affect it, such as soil, light, water, food and temperature.

Cuttings from one plant are clones. They have the same genes as the parent plant. In many bigleaf hydrangeas, acidic soil lets the roots take up more aluminum, and the aluminum helps make the flowers blue. In less acidic soil, the flowers are pink. Gardeners measure this with soil pH, a number where lower means more acidic.

Soil pH (Virginia extension guide)	Flower color in color-changing hydrangeas
About 5.0 to 5.5	Blue
5.6 to 5.9	In between
Above about 6.0 to 6.5	Pink

Some kinds of bigleaf hydrangea stay pink or white in any soil. The genes set which colors are possible, and the soil helps decide which one appears.

People show the same pattern. Your height depends on many genes and on food and health while you grow.

Elaborate: cut tails and counted traits

Starting in 1887, the German biologist August Weismann cut the tails off white mice and let them breed. He did this to the parents for five generations and looked at the young’s tails.

Group of tail-cut parents	Young born	Young with short or abnormal tails
1	333	0
2	237	0
3	152	0
4	138	0
5	41	0
Total	901	0

Source: August Weismann, Essays upon Heredity, English edition, 1889, pages 431 to 433; white mice bred from 1887 to January 1889. Normal newborn tails were 10.5 to 12 millimetres long. He gave totals for each group, not a measurement for each mouse.

Trait	Allele	How common (per 100 people)	Who was counted
Small extra finger beside the little finger	Dominant (thought to be)	0.035	Babies the study grouped as White, Boston hospital records, 1972 to 2012
Small extra finger beside the little finger	Dominant (thought to be)	0.91	Babies the study grouped as Black, same study
Rh-negative blood	Recessive	About 15	People of European ancestry, rough estimate from a 2005 reference book
Rh-positive blood	Dominant	About 85	Same group (100 minus 15)

Sources: extra-finger rows (the study counted extra digits on hands and feet, mostly hands), Holmes and colleagues, Birth Defects Research (2018), 289,365 births, stillbirths and pregnancy terminations for fetal anomalies; Rh rows, Laura Dean, Blood Groups and Red Cell Antigens (2005).

1. Add the young in groups 1 to 5. Does your total match the table? What fraction had short or abnormal tails?
2. If cut tails were inherited, what would you expect to see in groups 4 and 5?
3. Is the extra finger common or rare in both groups? Compare it with Rh-negative blood, a recessive trait.

Design problem

Plan an investigation to test whether light changes a trait in plants with the same genes.

One way to do it

Question: Do cuttings from one plant grow to the same height in bright light and in shade?

• Plants: 10 stem cuttings from one coleus plant, so all have the same genes.
• Independent variable: light. Put 5 pots by a bright window and 5 pots in a shady corner of the same room.
• Dependent variables: height in centimetres and number of leaves, measured each week for 4 weeks.
• Constants: the same pot size, the same soil, the same 50 millilitres of water twice a week, and the same room temperature.
• Control: the bright-window group, which gets normal growing light.
• Repeated trials: 5 cuttings in each group; find the mean height of each group.
• Safety: wash your hands after handling soil. Never put plants in your mouth; some houseplants are harmful if eaten.

If the groups differ, the environment changed the phenotype, since the genes were the same.

Evaluate: check your thinking

The two hydrangea cuttings have the same genes, which allow blue or pink. The soil in each garden decides which color shows. The small extra finger is rare even though its allele is thought to be dominant, because dominant tells how a trait shows, not how many people carry the allele.

Science practices you used: asking questions about a puzzle, planning a fair test with a control and repeated trials, reading data tables, and making a claim with evidence and reasoning.